Firmware v7.21.2008 was first distributed with the HP SDK, and is not currently known to be provided on any unmodified HP-20b.
HP software v7.21 (v7.21.2008) – 21-Jul-2008:
File: HP20b original ROM.bin (in SDK)
MD5 checksum: 07fa8a65815ee2cb52aa284152d51c5d
cksum: 3999079144 131072
Self test checksum: 126.126
Firmware v6.24.2008 seems to be the most current version floating around unmodified HP-20b's in the field.
HP software v6.242 (v6.24.2008) - 24-Jun-2008:
MD5 checksum: 7387a2cabec7dc53ca4224537d52327c
cksum: 857666795 131072
Self test checksum: 213.213
MD5 checksum: 08e1b75924aca69ee0be33ee482ffcf9
cksum: 4280875116 12288
Self test checksum: Not applicable
Images were downloaded three times (JTAG clock rate of 380kHz, 100kHz, and 58kHz). All three images were bit-for-bit identical. MD5 checksum and cksum values were for the entire 128kB flash (or 12kB ROM, in the case of rom.bin).
(Someone else please verify that the dumps are correct…)