That strap adds a sporty casual dimension to the Railmaster which is why IMO it is so versatile. I would love something like that which fits in to the OEM clasp for my RM XXL. Unfortunately the 24mm to 19mm dimensions mean it would have to be custom made. I use this Heroic18 24/24mm for a similar casual purpose and quite like it but I'd love something like your Cordovan there.

I could not agree with you more about the Railmaster being one of the best modern Omegas as well. It is just so versatile, unpretentious and functional. In fact I think I'll go get mine out of the safe and pop it on today.
Regards,
Zac