they have different movement (1030/1530), hence different dials, cases, etc...
More reading here:
The 6536 is probably rarer.
Cheers
Jeff

the reference 5508 is the upgrade of the 6536/1 with the improved cal. 1530: this movement has a butterfly rotor in the late 1950's and then the full rotor that we are used to since the early 1960's.
The cal. 1030:
The cal. 1530 butterfly:
A 1520 with full rotor:
I think that these straps have been designed only recently.
Cheers
Jeff
Cheers,