Anyone recognize this Nikon intermediate tube?

[PeteM]

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This intermediate piece, 70mm in height, came with some Nikon Labophot/Optiphot era accessories from a pathology lab. Does anyone have an idea of what it is? Has standard Nikon head dovetails fittings to and bottom. Marked "Nikon" but no other identifying letters or numbers of any kind. Topside optics have a large diameter. Bottom side optics on the long stem reach further than I would expect into a normal nosepiece.

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