The 16 mm does have a larger clasp and it's the same size as the clasp on the monogram strap above that Septamom posted a link to.
The 12 mm and 16 mm refers to the width of the strap itself.
View attachment 3895357
From L to R:
-- 12 mm strap that came with Eva. The Alma BB strap is the same 12 mm width.
-- 16 mm epi noir strap. This is similar to what Lillis posted above at the beginning of this thread, just in epi instead of ebene.
-- 20 mm monogram strap, the same as Septamom referenced.
I don't have a Graceful but have Delightful PM and GM and the middle and last straps' clasps don't open wide enough to fit DL rings without forcing and possibly scratching.