Ok I guess no one knows much about these either. I've found a little info about them both though. The 6X heads were more of the performance nature in early 70's, most had the 2.11/1.66 valves and some had press in studs but most had screw in studs. There is a machined spot on the head that has a single number stamped or a digit with a letter after it. The Letter after the single digit started in later years. The 4X heads were thrown to the side alot because they were labeled as a poor performer, but had the same features as the 6X heads. The heads with 1H, 3H & 7H stamped on head had 2.11/1.66 valves, but the 4H stamped 4X heads had 1.96/1.66 valves and was typically found on 2 barrel automatic cars. All the 3H stamped 4X heads had screw in studs and the 7H heads had both screw in studs and pressed studs.
The 6X heads had a few different compression ratio's #4 stamped heads had 91 cc chambers, #8 had 98/101 cc chambers (most was on 400's), #11 had 114 cc chambers (mostly found on 455's). Not sure if this helps anyone else, thought I'd put it out there in case.