ID numbers on 383 block

I am trying to ID a 383 I have. It is in a 68 A body. The block casting is 2468130-9
Also the stamps on the pad under the distributer has D383 12 28 R
also has the 2 threaded bosses on the front of the drivers side above the oil pump.
From what I read its a 68 block cast 12/28 ? What would the R stand for? Was there a HP stamped for hi-pef like the 440s?
Thanks in advance