The HM-133, HM-133S and HM-133V are variations on each other for particular radios. While Icom has updated the internals of the microphones and modified the key labeling to tailor it to a particular radio, from a protocol standpoint they all operate the same. The first of the series seems to have been the HM-133S, which (according to the HM-133S Users Guide), is an evolution of the Icom HM-98.
Please see the Icom HM-98/HM-133 Internals page for details on the protocol and such.
The three HM-133 variants are:
- HM-133 (IC-208H, IC-2820H, IC-2725E, IC-2720H)
- HM-133S (IC-2100H)
- HM-133V (IC-2200H, IC-V8000)
The HM-133S is electrically identical to the HM-98, but with a different key layout. The HM-133V comes in two versions, one that is electrically equivalent to the HM-133S, and one that is electrically identical to the HM-133 (which I call the HM-133V(A)). Outwardly there is no way to tell the difference between the two, and functionally they are all identical, differing only in key labeling.
Curiously, in the HM-133V(A) and in the HM-133, there is a 24L02 256x8 EEPROM. This might be used by Icom to give the microphone a 'profile' of some kind. One might hope that it's used to program the PF1 and PF2 keys, but those are actually key sequences sent to the radio. At this point, I have no idea what the EEPROM is actually used for.
The older microphones use a μPD7564AG 4-bit microprocessor with a 1KiB masked ROM. The new microphones use a μPD789071, which appears to be a Z80 core with 2KiB of masked ROM. Datasheets for each of these parts may be found on the Icom HM-98/HM-133 Internals page.