It is an P5 head on an R4 block. Was originally developed by Mopar for Pro Stock Truck with Rick Waters and others. It was the replacement for the W8 stuff they had been using. Cavallo was originally an RWR customer til be went to Book. Awesome parts EXCEPT there are no blocks and you really need the billet head otherwise its a crap shoot on whether they will be good or not. There are a few P5 deals running here in Comp as well, most at 358". It is a very good platform unfortunately with little aftermarket support. Billet heads are produced but blocks are a big issue. Other than possibly a billet block. There was also a "clamshell" version of the P5 head. There is a pair in the classifieds here. There are a few differences in that head and the later P5 stuff. The later heads looks more like an asymetrical 99 Hemi head, which is still used in Pro Stock along with the symmetrical version.