This is the standard Royal Microscope Society size - 1.27" or 32.26mm nominal - so the rare large professional size(and likely quality). 16x. I suspect it is Watson as it came with many other watson eyepieces and the card on the microscope was from watson ( card is in the photos for information , but NOT included in sale) but the eyepiece isnt marked as Watson. It has a removable eye cap and could use a clean up which wont be hard as it disassembles readily. 

See my other listings for 2 other RMS eyepieces (at time of listing)