Nikon Alphaphot-II YS substage condenser with 33mm filter insert retaining ring (filter not included). The Nikon product number is MCL51100 and is specifically designed for the alphaphot-II YS because it inserts from beneath the condenser fork as opposed to clamping on top as is the case with the labophot, optiphot and the alphaphot-I.  This is the newer style of condenser because it has a n.a. scale on the diaphragm control lever and can accommodate lenses with an n.a. of 1.25. The condenser is new, discontinued stock, still in original packaging no longer available from Nikon. The sale is for one each.