I'm looking for the same information as @Alex_TA . My boutique wasn't sure and since it's a special order (non-returnable), I'd love to see if someone has tried it on their new Speedmaster 3861.
I put the same exact strap on an older model 3590.50 as well as on a 2018 1861. Fit like a glove. I don't think they changed the case that much did they?
Ditto, just put the Omega Rally Strap on my 3861. Fits like a glove, really loving the way it hugs the Bezel without any gaps. Definitely a keeper for me.