Thanks everyone!
This one doesn’t get as much wrist time as it deserves, but it’s an obvious choice for the beach. And today’s glidelock bracelet fits the 5513 like a charm. I have the original bracelet too, which is cool and reference-appropriate but less comfortable. ...
I think that switch to letters was in 1987
so this is one of the last crop of number-only serials. Or so I read on the internet!