GPS 12 Versions

My first GPS was a 12 I used until Etrex time and it was put in the cupboard. I retired, did other things and had need for a gps again. Dam! the Etrex was quirky which I put down to buttons being frequently unworkable by - I think - the shock absorbing material deteriorating.
Anyway, I had 2 Garmin 12's in the cupboard. Fired them up. All Ok -except the date rollover of course. Fixed that OK.
One has the 3.50 versions of firmware, the other has the 4.50 versions
I cannot get my win10 PC to update the 3.50 machine. Windows will not run the software and I cant get a "compatibility" mode to fix it.

Now, in the dim dark past, I think the two versions were two different and incompatible beasts but cant recall the detail.
Do any of you oldies reckon I can load the 3.50unit with the 4.50 software and have 2 identical working units or will it brick the 3.50 version ???

 

Thanks Nuvi I had downloaded both the 3xx and 4xx versions. The 4xx version works fine but the 3xx will not run under win 10. It is a different format with a Hex file rather than the .bin files of the 4.xx and win 10 tells me "This app cant run on your PC". Various Compatibility modes as I said doesnt help
Hence the question.
I still have 2 x 12's working just different update levels for what thats worth

 

Both are labelled GPS12 on the backplate where the serial numbers are as well as on the front screen panel
There is nothing I can see to indicate Manufacture date