Yes, that's the measurement I was referring to.
Unfortunately, I don't know of any OEM adjustable clasps that will mate with that bracelet due to the 11mm middle link. There are links with a middle link width of 9.5, 10 and 10.5 that will work. But 11 is too wide.
Below is a thread from WUS that discusses a lot of cross compatibility of OEM clasps. I've participated several times in it. Perhaps there's something in there that's useful to you. I've never owned a Railmaster with 20mm lugs, nor have I owned any 41 or 41.5mm Aqua Terras so I've not been looking specifically for that solution. I have, however, owned every generation of the 36.2/38/38.5 Aqua Terra but those have different bracelets compared to their larger variants.
If memory serves correct, the pre-2018 Seamaster Professional 300m end links/bracelet will fit the 20mm lug width AT's/Railmasters, and there are adjustable clasp options for those. Several are noted in the thread below. But that would require buying a new bracelet, not just buying a new clasp. And I'm not 100% it will fit, so if you want to go that route I would make sure they are compatible.
I wish I could be of more help. I've converted all my AT's and SMP's to an adjustable clasp because I live in a hot climate with a lot of wrist size variance due to the difference in temperature between indoors and outdoors. Plus, I drink a lot of water in the summer due to the heat and I walk outside for about 1.5 hours every day. Adjustable clasps are must for me.
https://www.watchuseek.com/threads/adjustable-clasp-conversion-compilation.5396509/
