1968 Service Manual states 9.2:1 compression for my Coronet and I measured how far down in the hole they were (100% original car) and they were just about 0.028"/0.030" with a steel gasket. I think that would get you very close to 9.2. The Sealed Power Pistons put you a disgusting 0.090" in the hole.
BTW, I ran SCR with a .028" in the hole piston with 4 cc's of eyebrow volume, and a .023" thick head gasket, and typical 675 head chamber volume of 68 cc's; it comes up 8.9:1.
To support your info, I just looked up some piston info in the '69 and '73 FSM. They list 'piston length'.... top to bottom of skirt. For '69, it is 3.21" , and for '73, it is 3.47" . So, clearly there was a production change in the pistons within that time frame. Sadly, they don't list compression height, or give any reliable pix. Some of the 273 pistons tops were only a few thousandths in the hole at that time (I have a set), so your finding seems consistent with the very early engines being better set up for CR.
Thanks for the info... yeah, look at those pix! Very early 318 values just shot up LOL...
