Welcome to the forum. The 3861 Speedy Pro bracelets do not have on-the-go, adjustable clasps.
So it carries forward the previous generation bracelet's approach of having 2 different slots on the clasp, a few millimeters away from one another. Which you'll need a tool to adjust. We also saw from Spike's posts that the final link looks a bit off, when using the outer-most adjustment slots.
There's only been a very small handful of manual-wind Speedmasters that a pusher-style, fine-adjustment clasp. The 020STZ006270 bracelet on the Apollo 17 45th Anniversary, the clasp on the Tokyo 2020 Speedmasters, the 60th Anniversary, are the only 3 that jump out in my mind. Of course, the clasp on each of those would incongruous, with the 15mm taper of the new 3861 bracelets.
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