For info, part number 51782985 is an early software version of 51867767 so either can be fitted. I currently have 51782985 fitted to a 2008 500 and when contacting Fiat for a replacement they quoted me for the 51867767 version. There is also an interim software version 51848079 which should also be a suitable replacement.
(These are all metal cases.)
Be aware that there was a DVLA recall for the airbag system on the early cars; IIRC the fix involves reflashing the firmware, so I'm not entirely surprised to hear that Fiat also updated the part numbers for the airbag ECU.
If the airbag ECU firmware isn't the correct version for the car it's fitted to, the airbags may not deploy correctly in the event of a collision. In the worst case scenario, this could be be the difference between life and death for the vehicle occupants.
Given that some, but not all, of the early ECU's will have been reflashed as part of the recall, there's absolutely no way of correlating part numbers with software version on a secondhand unit.
So far as I'm aware,
MES can't reflash the airbag ECU, nor is there any readily available version information or reflashing software in the public domain.
Putting the airbag warning light out is easy compared to establishing with certainty that the correct airbags will fire with the correct timings and sequencing in he event of a crash.
There are sound reasons why the code of practice states that salvaged SRS components, including airbag ECU's, should never be reused.