Recovery volume does not show up in Startup Manager

Question

While upgrading a MacBook Pro from OS X 10.11.6 El Capitan to macOS 10.13 High Sierra in a step-by-step procedure, I created a clone of the Recovery HD volume. Unfortunately, this newly created clone does not show up in Startup Manager's selection list. Details see Problem section below.

Hardware-Software-Environment

MacBook Pro Early 2015 running OS X 10.11.6 El Capitan.
The internal HD is actually a 250 GB SSD comprising partitions EFI, Macintosh HD and Recovery HD. There is no encryption enabled on any of these partitions. External 2 TB HD connected via USB 3.0 Bus; later on called target disk.

Task

Create a bootable clone of the Recovery HDpartition on the target disk. Provide a robust and simple command line based procedure. Wherever possible, avoid fiddeling around with the GPT entries.

Solution Attempt

1. Apply diskutil resizeVolume to create a new 1.0 GB partition on the
   target disk carrying a JHFS+ file system. To prevent subsequent naming
   confusion we 'baptize' the newly created partition to Recovery HD so
   that the name of the partition and the corresponding GPT label will agree.

2. Run asr restore to 'populate' the new recovery partition.

   asr restore --source /dev/disk0s3 --target /dev/disk2s9 --erase
   

   where, in this case, disk0s3 is the disk id of the recovery partition which is
   associated to the startup volume while disk2s9 is the disk id of the 'new'
   recovery partition on the target disk.

3. Run asr adjust to set the appropriate type to the 'new' recovery partition

   asr adjust --target /dev/disk2s9 --settype "Apple_Boot"
   

4. diskutil info output for the 'new' recovery partition:

   Device Identifier:        disk2s9
   Device Node:              /dev/disk2s9
   Whole:                    No
   Part of Whole:            disk2
   Device / Media Name:      Recovery HD
   
   Volume Name:              Not applicable (no file system)
   
   Mounted:                  Not applicable (no file system)
   
   File System:              None
   
   Partition Type:           Apple_Boot
   OS Can Be Installed:      No
   Media Type:               Generic
   Protocol:                 USB
   SMART Status:             Not Supported
   Volume UUID:              AA10BAA6-C29B-37A6-BA1C-0EACCFD304C9
   Disk / Partition UUID:    2B1B46B7-8068-493F-A8CC-475EFECD89EA
   
   Total Size:               1000.0 MB (999997440 Bytes) (exactly 1953120 512-Byte-Units)
   Volume Free Space:        Not applicable (no file system)
   Device Block Size:        512 Bytes
   
   Read-Only Media:          No
   Read-Only Volume:         Not applicable (no file system)
   
   Device Location:          External
   Removable Media:          No
   

   gpt -vvv show -l /dev/disk2 output (snippet):

   gpt show: /dev/disk2: mediasize=2000365289472; sectorsize=512; blocks=3906963456
   gpt show: /dev/disk2: PMBR at sector 0
   gpt show: /dev/disk2: Pri GPT at sector 1
   ...
   gpt show: /dev/disk2: GPT partition: type=426F6F74-0000-11AA-AA11-00306543ECAC, start=1331274184, size=1953120
   ...
   gpt show: /dev/disk2: Sec GPT at sector 3906963455
       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
    ...
   1331274184     1953120      9  GPT part - "Recovery HD"
    ...
   3906701272      262151         
   3906963423          32         Sec GPT table
   3906963455           1         Sec GPT header
   

Problem

With the external HD connected, invoke the Startup Manager holding down the ⌥ Option key on starting or restarting the machine.

The Startup Manager does not show the newly created recovery partition in the list of volume icons representing those partitions it considers bootable.

Question: What is wrong with the above solution attempt trying to create a bootable recovery partition? Any advices and suggestions are welcome.

Additional question: Can it be that this issue is induced by a side effect of the System Integrity Protection feature that has been introduced in El Capitan? See item #4 in the Sources and Footnotes section below.

Observations and Tests

1. Observation: We start from a properly created clone of the built-in startup volume, thus it consists of a Macintosh HD and a Recovery HDpartition, hereafter invoke the Startup Manager, the clone now appears as a (single) icon labelled Macintosh HD.

   If we rename solely the Macintosh HD to, say, macintosh_hd_10_11_6_yymmdd the clone appears as a pair of icons labelled macintosh_hd_10_11_6_yymmdd and Macintosh HD, where the latter denotes the recovery partition.

   If we rename both, theMacintosh HD and the Recovery HD to, say, macintosh_hd_10_11_6_yymmdd resp. recovery_hd_10_11_2_yymmdd the clone appears as a (single) icon labelled macintosh_hd_10_11_6_yymmdd.

2. Observation: Again starting from a properly created clone of the built-in startup volume, we 'purge' the Macintosh HD via diskutil reformat while leaving the Recovery HD untouched. Now, the Startup Manager does not show the 'left-over' recovery partition; before, the clone of the built-in startup volume appeared as an icon labelled Macintosh HD as described above.

3. Tests: It doesn't achieve anything ..
   .. if we carry out the above processing steps in Recovery Mode.
   .. if we modify the above processing steps in so far that we start from another (working) Recovery HD partition clone instead of the one associated to the current, built-in startup volume.
   .. if we modify the above processing steps in so far that we, at first, create a disk image
   of the Recovery HD partition and subsequently run asr restore taking this disk
   image as source.

4. Test: Before executing step #2 of the above processing steps we
   disable SIP. (Run csrutil disable in Recovery Mode; don't miss to re-enable SIP when you're done!)

   Disappointingly this did'nt fix the issue.

Sources and Footnotes

Changing a Volume's "Device / Media Name"
Discussing the relationship between partition name and label resp. volume and device/media name.

Recovery HD Is not showing up. Is it because of FileVault?
Contains an insightful discussion about the Startup Manager's label trickery.

https://bombich.com/de/kb/ccc5/frequently-asked-questions-about-cloning-apples-recovery-hd-partition
A very useful text from the Bombich Software knowledge base introducing the term of an operational Recovery HD.

https://www.macobserver.com/tips/deep-dive/macos-recovery-partition-without-reinstalling-macos/
Presents the Recovery Partition Creator tool; mentions that System Integrity Protection (SIP) needs to be disabled in order to clone the recovery partition in El Capitan.