Here's a pretty good list of possible serial number sequences:

https://coolbload453.weebly.com/six-cylinder-chevrolet-engine-serial-numbers.html

I don't see the F55 suffix number listed, so it's possible you have a replacement block with the high water pump mount position that was cast after the switch was made to the low water pump position. The lack of a casting number behind the fuel pump reinforces that possibility. John Milliman's 261 engine is one of those- - - -a high pump block with a 1959 casting date code. That little detail almost derailed a rebuild when we ordered the wrong main bearings, based on the belief that we had a 1954 production block! Lots of little details can be overlooked when working on a 70 year old engine, especially one that's been rebuilt a few times!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!