I currently have just one spare antique Watson brass 41mm dia thumb-wheel from a recent refurbishment project.  This is essentially just one coarse focus thumb-wheel, there's no pinion shaft/gear.  The item will date to 1913 and should fit monocular Watson Bactil, Standard and maybe some other models from that era - please check measurements with seller prior to ordering if you're unsure.  If your brass-era Watson has got a damaged coarse focus thumb-wheel, if it's missing altogether or you just want to swap it, then you can quite easily freshen it up/replace with this item.  The fitting is a friction-fit to the pinion shaft and there's various measurements shown in the listing photos.

The item will be well wrapped and sent by Royal Mail.

Thanks for looking.