289 hipo date code question

someone aske this on another forum, wated to see if anyone could answer this for him:

I recently came into possesion of a 1979 mustang. upon further inspection, i have come to the conclusion that the engine is out of a 1963 ford fairlane 289 hipo. funny enough, when i took the heads to a local machine shop i recieved a call the next day from a guy that worked there. he informed me that one of the guys at the shop was going to swipe my heads and replace them with an old set. needless to say, when i arrived my heads were still there. i gave the guy that called me 20 bucks. i figured it was the least i could do. anyway, the heads have date code 3d24. don't know what the d stands for. also, on the bottom of the heads there is c30e, F: a big C with an f in it. one head has 20, the other is 21.

Any help would be appreciated.

Thanks all,
Mark