Come home from the fleamarket, puzzled...
Found a box designated as "Elmo Super Zoom Converter FP-C". (Thus some cine stuff.)
Inside the box one of those hard plastic, transparent domes to screw a lens onto ist base.
Base has got a M25x0.5 thread.
Screwd onto the base is a aluminium barrel looking like the front part of a projection lens. Though extremely well finished.
At the front a designation ring: "Elmo Projection Lens F 1:1.3 f=25mm"
Inside it is machined as plain flat cone (25-30mm dia.) with a perfect flat black finish.
For the rest: NOTHING