The XP2008UB is a 10-stage, 1.5-inch photomultiplier from the Philips / Amperex / Valvo XP family. Amperex was Philips' U.S. name on these tubes. Photons hit a photocathode; dynodes multiply the electrons; a pulse comes out the anode. In the MicroBasement it sits with the EMI and DuMont PMTs: same job, later European production.
Philips in Eindhoven built photomultipliers for nuclear physics, medical scintillation, and industrial counting. Amperex Electronic Corporation sold the same line in the United States. Valvo was the German Philips tube brand (XP2008 shows up in Valvo 1972 lists). The XP numbers replaced earlier PM-prefix types. XP2008 is listed as replaced later by XP2071-class tubes. Fermilab notes from 1993 still treat XP2008 / XP2012 as 1.5-inch, 10-stage candidates for calorimeter work (UA2 heritage, CDF studies).
Head-on or near-head-on 38 mm class envelope, 10 dynode stages, duodecal B12A 12-pin base. Typical seated size about 38 mm diameter and 122 mm long. "UB" on Philips/Amperex PMTs is a selection or processing suffix on the base type, not a different diameter family.
These tubes run from a high-voltage divider. They are display pieces here, not a project to power up.
DuMont K1303 is the rugged 1950s American 6-stage look. EMI AU8303 is the venetian-blind lab bottle. XP2008UB is the 1970s Philips 10-stage 1.5-inch can that physics groups still had in the drawer in the 1990s. Same idea: turn a flash of light into a countable pulse.