receivers for AKM 63 vs AK????

Ok, I was looking at flats for a couple of builds I'm planning. I came across a web page selling a flat for the AKM 63, but I think it was for an underfolder not the fixed stock. Can anyone tell me if there is really a differance in the two (AKM 63 vs AK) other than the underfolder part? The page charged two times the normal price of 80% flats for this one and said this is the one you have to get for the 63. I compared the flat I have to the picture and could only see a few holes in different places. My flat has the trunnion holes pre drilled too....
thanks
Greg

4 more days before I get to go pick up my new GP WASR 10 (rebuilt AKM double stack - by Century)........... [postal]