OK, I made a couple of token additions to id31.py; eg:
  1. Copied class "ID31Radio" to "ID51Radio".
  2. Changed the .ICF file size, signature, and endframe.
  3. Changed the rf.valid_bands to [(108000000, 174000000), (400000000, 479000000)]

I then imported my ID-51 .ICF file, and most of the memory lines imported correctly !!  Ditto for reading from the radio (serial port COM220) !!

Since some of the frequencies didn't import correctly, I suspect that this is the same (or similar) issue as reported in bug 559 for the ID-880H.  There are other "Gotchas" as well, like not knowing about AM mode, and it appears that the "Skip" bit has moved ...