5N06 etc. seems to me a very familiar style of voltage / current rating with some Power MOSFET makes. So MPT could just be a manufacturer prefix.
And searching on just that in mouser, find a lot:
https://www.mouser.co.uk/Semiconduct...d=5N06&FS=True
https://www.mouser.co.uk/Semiconduct...g?keyword=5N06
But not sure what package original ones is in - can't find much on that part in Google - to narrow down a bit.
I'm also wondering why there'd be a MOSFET in the chopper transistor's emitter circuit - normally that goes straight to the -V HT DC or sometimes via a current-sense resistor.