the 308 and 5.56 use the same buffer tube (receiver extension if we want to be proper) but the spring and buffer are specific to the platform. You will need the 308 rifle buffer and rifle spring for that PRS.
As far as A1/A2...the only difference is a small spacer. An A2 stock is longer, so a small spacer sits on the end of the standard milspec rifle buffer tube to take up the extra space and a longer screw is used. A1/A2 only refers to the stock length and doesn't play into buffers or springs.
The PRS is A1 length so 1) standard rifle length tube and 2) 308 specific rifle buffer and 3) 308 specific rifle length spring is what you need. No spacer needed. The PRS should come with a screw, but if not an A1 length screw will work.
If I just told you a bunch of stuff you already figured out, sorry :) might help someone else doing a search down the road.
This is assuming DPMS patterned rifle. There is no standardization so if were talking AR10/larue/sig/RRA/etc all bets are off lol.