That's an odd picture, took me a second to realize why. (the stock is actually on upside-down in relation to the trunnion)
You should have/need one long through rivet and 4 short rivets. Of those 4 short rivets... one will be a little longer than the rest (usually) to allow installation of a rail. If you don't plan on using a rail, use a regular length short rivet. All rivets are traditionally finished with flat heads, with the exception of the head of the through rivet on the right hand side (starboard). That should be round.
I actually just finished up an AK74M build with one of their 101 stocks and trunnions. Keep in mind that if your going with a DCI receiver, the angles for the rear trunnion are slightly different (very slight) because they were cut for a bulgarian trunnion of that type.