Few questions in this thread, so I'll provide what information I can...
The acrylic crystal on the 3861 model (31030425001001) is the same crystal that has been on the previous Speedmasters, all the way back to 1968.
Dial for the 3861 acrylic crystal is 31.2 mm.
Case back seal for this watch is 088NS5090 | SEAL O-RING D34.5 D36.1 Ø0.8
If I look at a previous Speedmaster, say the 35705000 and the 31130423001005, they use the same parts below:
It uses the same acrylic crystal as I noted.
The dial is also 31.2 mm.
The case back seal is 088NS5140 | SEAL O-RING D34.5 D36.5 Ø1
Looking at the sapphire 3861 (31030425001002)...
The crystal is 34.2 mm in diameter, and then there is a crystal gasket on top of that, so the opening in the case is larger than the crystal.
The acrylic crystal is 34.11 mm in diameter.
It would be too small to fit directly into the sapphire case.
The case back seal diameters would suggest that the case backs may be swappable between the older designs and the 3861, but of course one would have to try it to be sure. It appears that the only difference is that the 3861 uses a slightly smaller cross section seal.
Click to expand...