I struck a bit lucky on eBay the other day. A brand new, unused micro-adjust push-button clasp for the 3861 appeared, so I popped a bid on it and won.

Mounted it today. It makes all the difference having that slight (5mm) adjustment for my wrist's variance in temperature.

I will say though that the bracelet on the 3861 is massively fiddly to adjust. The screws are so tiny, smaller than I've ever encountered on a bracelet. It's beautifully made though, as is the clasp.

If you have a 3861 without the micro-adjust, it's definitely worth sourcing one.