Mick,
I do have makings on the caseback of my 11630 GMT, same size as yours, but not quite like yours. Mine is 'C' followed by 3 numbers (I forget what they are).
This signifies that it was made for the Swiss domestic market and distributed by Heuer’s Swiss distributors Cuanillon. Hence the 'C'.

Maybe yours are codes for another country?

Stewart

: My recently acquired 1163 GMT shows a small number engraving on the
: back:

:
:

: A closer view:

:
:

: I would appreciate any information or hint about the meaning of
: "37-6". Did anyone see similar engravings, probably of
: other numbers?

: Thanks,

: Mick