Chevy 1962 235 - code next to the distributor: F0511A

Does anybody have a link or know what the 'A' suffix letter means?

I am trying to find out what this engine came out of.
It has hydraulic lifters, so I assume it's from a car with powerglide.

The head is not the 848 but the 913 with lower compression ratio used in cars, according to the casting numbers index on this site.
Why would the ratio be lower for automatic transmissions and is this the head with the larger intake valves?
What stock comp ratio does it have?

And one more: What is the compression ratio of a late 261 with 848 head, not milled?

Thanks for any input...