You are right, I have looked at boot.bin form several firmwares and found the same commands there. I'm able to run boot.bin on 37xx, just need to save it on SD as \Garmin\Updater\1101\Ldr.bin

I have made some tests. Ldr.bin runs just after booting, probably is executed by real boot.bin. Ldr.bin parse Update.txt and execute all commands written there. At the end it write 2 files in its directory: last_id.bin and Update.log. When these files are present, Ldr.bin is not executed again.

There are usable commands for Ldr.bin. We can read and delete files. For example command "del,0:/.System/gmapsupp.img" can remove damaged map from nuvi. We don't need cure firmware anymore

We can copy backup files from SD to nuvi and restore it after formatting

I think commands like fftl or copy_image can even restore damaged file system. This needs more investigation.

I found that commands are present in 3 years old firmware. It is unbelievable that nobody reported this feature before. Just imagine how many problems could be solved that way...