This is the standard Royal Microscope Society size - 1.27" or 32.26mm - so the rare large professional size(and likely quality). The C designation for 8X shows it to be the older model. I suspect it is Watson as it came with many other watson eyepieces and the card on the microscope was from watson and C was marked as 8x ( 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)