It is 16mm. Steel. Asking price is 59€/$.
Here is the back, seller says this is written on the backside: AC EP INOX (than a sun as a symbol) SWISS

 

Looks geniune, as far as I can see.
The price is very high for it is the most common from the 60's.
Best
Mounir

 

I have an almost identical one, so am 99% certain it is authentic, but mine doesn't have anything stamped into it.

 

Are there buckels that match Speedmasters from 57, 84 or and 2004?
Or do I just take a normale buckle that is available from any AD?
No clasps please, no steel bracelets.

 

It should be stamped with "acier inox" and "swiss" and a sun on the back side.

 

The problem will be to find a strap that is on one side 19 or 20 mm and 16 on the other, I mean for the 84 and 57.
Did the Speedmaster exist in 57 ?

 

Straps in 20 to 16? Plenty of it.....try www.bulangandsons.com

 

Buckle looks fine.
Expensive, but try to order a new/modern one from an AD!