I made a point of getting my hands on 3966 over the weekend, and if it's any help, it's one of the prettiest bows in the collection! The WoW measurements are quite far out (as with most of the bows) and it's actually very small in the hand, but the grain is exceptional, far too tight to count.
It's one of the better finished bows too, with virtually no sign of the faceted edges that are the surface finish of most of them - it's been gone over with a sharp edged tool like a scraper to take all the corners off.
The bow itself is very round in section, almost circular and the tips are absolutely miniscule. 10mm at the start of the horn stain with tiny grooves in the wood where the string groove was cut through the horn.
There's next to no sapwood either, about 1.5mm in the middle.
What's funny is that on such a pretty bow, with such attention to detail on the finishing, the "bowyers mark" is an ugly great cross scratched into the side with something thick and blunt. Imagine a ballpoint pen which has run out of ink and you'll be close.