You know they must come in both sizes. I have the same ref. buckle & it's definitely 18mm. Had a Leather Strap made for my Speedy, 20/18mm with the STZ010566 11IJB.
It is a fabulous buckle. Mine is on eBay now. I happened on this thread researcnhing for the listing. Selling bec I sold the Speedy & bought a GS. The GS 18mm Buckle is quite nice too
Click to expand...