Those are an early set of the Kenne-Bell "Stage 3" heads. Brand new is pretty cool. I believe the pair I bought from them in the early 80's were $600. Basically factory replacement stock heads that they had well ported. Original Stage 1 swirl polished valves too. $2500 might be ok if you really need good iron heads but $4500 is crazy since there is nothing to make them stand out from normal heads on the outside. Gessler checked out the flow on one of my KB S3 heads way back and although they flowed pretty good the set he did up for me flowed noticably better.
I believe these would have been " kenne bell stage 3" heads that was service replacement heads that was ported out
If your wanting to do a KB car then this might be something worth the while.........but if what you want is performance the money can be better spent
Price coming down. Looks like $2G would get them. If I really needed a set of good cast iron heads I'd pick them up. Who's porting cast iron heads these days anyway? https://www.ebay.com/itm/NOS-GM-197...303677520859?_trksid=p2385738.m4383.l4275.c10 KB MK3 cam price is now reasonable. I was running the MK3 and MK3H back in the day and they were great cams. Funny thing is the cam in my '76 was a Howards about the same as the MK3 except with a "milder" 108* LSA and the current cam is the same in a roller, 242*I/248*E @.050"/108* LSA. Both worked great and still are. https://www.ebay.com/itm/VINTAGE-NO...383709491935?_trksid=p2385738.m4383.l4275.c10
I still have my Kenne-Bell Stage 3 heads I bought in the early 70's. I have all the correspondence Jim Bell sent to his customers when he was unsuccessful when trying to cast Stage 2 iron heads. That's the reason the Stage 3 head came about. They were "S1" over-the-counter service replacement castings ported at CHK. He offered you your money back or you could opt for the Stage 3 heads. I have since upgraded mine with new larger SS valves but never touched the porting which was "hidden" by rough casting texture they did on them.