Basically every sheridan can be identified by its physical features. Look at a picture of a P12..that is what it is. It is a PMI1 direct feed with a longer barrel.
P68's came as AT (autotrigger) models, and SC stock class. The AT is a lot like a piranha Long barrel (back bottle, direct feed) but with the auto trigger assembly.
The SC is like a PMI1 stock class, but with a longer barrel (probably P12 length) and a longer feed tube. To me, they are too big and heavy. MAD's guns are configured in a more playable setup..PMI1's are overall the best starting point in my opinion.
The thing you need to think about is how much money it will cost for you to get the gun where you want it. Accessories add up quick and you might be happier just ordering a PUG from palmers with a .685 wedgited barrel. That is a sweet barrel size.
"I'll see it when I believe it"