This filter fits the Nikon Labophot, Labophot-2, Optiphot, Optiphot-2, Microphot, and TMD series of microscopes. It is a Nikon #78643.  The filter is new/ dealer demonstrator in its' original plastic container (not pictured).  The Blue Violet filter typically is used with Quinacrine Mustard and Di-Hydrochloride chromosome banding procedures. Thanks for looking!

DAPI Excitation Filter Block Specifications