28-29 coupe roadster rear subframe rivet. ?????????

[rust revival]

Hey again lots of knowledgeable people here, So this should be a 100% slam dunk for anyone that has done restorations
Can anyone tell me what rivet style and size goes in the rear sub frame support. I see they are waffle pattern on the back side, but have a high dome head, a 13/64 drill bit fits the hole, and they are not the same as the rest of the subframe rivets(Brazier Head) that's all the info I know
Any definitive answer would be appreciated greatly