I did some searching but only found that there is an adjustable clasp in 18mm that fits the Seamaster mid-size. I was wondering if it was somehow possible to fit this adjustable clasp onto the discontinued Speedmaster bracelet that tapers from 18mm at the lugs to 16mm at the clasp like the one shown in the picture below. I suspect there isn't an attaching link available to connect the bracelet to the adjustable clasp but wanted to throw this out there in case it does exist or there is another solution.
I've Seamaster mid-size with Speedy bracelet in front of me. It's not tapered at all. It has 18mm near the clasp. I don't see any way how this clasp could be fitted to 16mm endlinks from your bracelet.