It depends on the number of inputs your module has. From the 'Latest Firmware' thread:
2. This is the last firmware version which has the following naming scheme for the files in the archive:
megadrumppp_hh_yyyymmdd.bin for 22 inputs versions
megadrumppp_hh_32_yyyymmdd.bin for 32 inputs versions
megadrumppp_hh_48_yyyymmdd.bin for 48 inputs versions
megadrumppp_hh_56_yyyymmdd.bin for 56 inputs versions
where
ppp - AVR type. 32 - for Atmega32, 324 - for Atmega324(p), 644 - for Atmega644
hh - crystal clock in MHz. Atmega32 versions with a clock above 16MHz are overclocked, Atmega324(p)/644 versions with a clock above 20MHz are overclocked.
yyyymmdd - date(version) of the firmware.
If the one corresponding to your MCU/inputs won't work the fault is probably elsewhere.
If it ain't broken... fix it until it is.