This is a 2-inch (50 mm) head-on photomultiplier with venetian-blind dynodes and flying leads. The only mark on the envelope is AU8303, which reads as a factory or lot etch, not a catalog type. Construction matches the EMI / Thorn EMI 50 mm family (9536B, 6097B, 9531). The MicroBasement piece is display-only. It will not be powered.
A photomultiplier turns a few photons into a measurable pulse. Light hits a photocathode on the inside of the window. Electrons leave that surface and hit a stack of dynodes. Each slat multiplies the shower. The anode at the bottom of the stack is the output. Venetian-blind dynodes are the slatted shelves visible through the window. They are slower than linear-focused types and very stable, which is why EMI used them for scintillation counting and photometry.
Stock EMI 9536B tubes usually wear a B14A diheptal base and replace a DuMont 6292 / RCA 6342. This one never got that cap, or the cap was removed and the leads potted through the disk.
Without a printed type on the bulb, the card stays at "EMI-pattern" rather than a hard 9536B.
Tubes on this shelf are the other way of seeing electrons: power tetrodes like the Eimac 7203 / 4CX250B on one end, a photon counter on the other. Same vacuum, different job.