Leitz Microscope Objective - 6a ohne Deckglas Tubusl.215mm

This objective is unknown to me. It says E.Leitz Wetzlar 6a ohne Deckglas Tubusl.215mm. I can't find any info on it, so it is rare, probably. Or useless ;-)

Good used condition. Minor cleaning marks. Work distance is about 0.5mm.