On the MUT Moon 39, the original strap is 21 mm large ( between the lugs ) and 18 mm at the buckle.
On such an elegant model, I recommend a slightly tappered strap, such as the original one.
So, you can get all the buckles you want, but check that it is 18 mm if you want to use the original strap. You can also go for an aftermarket strap, respect the distance between the lugs, of courese, and opt for a 19 mm end at the buckle ( then, the buckle will have to be 19 mm large, of course ). But not more, as it is a dressy watch, IMO.
Best,
Nicolas. This message has been edited by amanico on 2011-09-05 23:20:11