P51 was the follow up of the P50.
From the Ipl's I can tell some differances in clutch and chain cover/chain breake. The 51 has some sort of chain breake in bar mount as some of these Jonsereds saws had to prevent kickback. I have not yet figured it out but I will have a good look later.
From the manual I see the 50 and 51 was both availeble with exoust heated handles.
This is questioned by me and others here.