The 848 head, as far as I can tell, entered production in late 55 for the 56 model year on the passenger car line. 1/2 npt for the electric temp sender. 54-55 would have been the 5913 head (not sure about 53). So it is entirely possible to see a head casting date of say November 55 on an 848 head. I have one with that date. But that would be a head cast for the 56 production. Frankly it makes sense in that for 55 the CR was advertised at 7.5 and if you look at both the 1956 car and truck supplements to the 55 shop manual, GM notes the introduction of more powerful engines, CR at 8.0, and the introduction of the 848 head. That being said, it would not surprise me in the least to find an early 56 motor with an 5913 head. I have to check, but I ran one of my old modified motors with a 5913 head I pulled out of a 56 car many many years ago. You have me curious now as to whether that was actually a 56 head or a 55 head.
In 58 or 59, those 235 motors were rated at 8.5 cr using an 848 head. I have never taken the time to sort out what the difference is between the 55-57 motors and the 58 and up motors to explain the difference. What I can say is that a good number of years ago, when Tom Langdon was still working at GM, he and I were tlking CR ratios and while we were talking, he looked up the actual GM specifications for the combustion chamber for the 848 head (among others) and it was 79.1. I don't know if its me (probably) but I have measured a couple of 848 heads and I never get a 79.1 reading, usually a little larger.