I don't know of any way to distinguish between low or full pressure by outward appearance. As far as I know, all of the blocks of this era had the holes for the bypass-type oil filter, which was an option for many years.
I'd say the LAQ code clinches the engine's identity as a '53 PG (assuming the serial wasn't restamped at some point). I'm sure heads were changed all the time, due to cracks or rebuilding, so the head would not be a reliable indication of the year of the original assembly. Also, the engine serial numbers had a different format starting in '54, with the use of suffix codes instead of prefixes.
