Thanks for the answer Mark, it clears up alot of things.

I don't have the watch in front of me (in the bank safe) but it is certainly not a hex back, particularly as you so rightly point out, Heuer never made any watch with a hex back, but neither is it a poly back*, it is a circular style back - I'll post a pic when I get the watch back home. The inside is Ed Heuer rather than Heuer Leonidas. This matches your theory of running out of the standard backs at the 54xxx range.

Do you agree also with the larger font on the serial number for the 53xxx range?

Shaun

* if we want to go super pedantic, then the circular back could be considered also a poly back with an infinite number of sides.