The diode seems to be a Sarkes Tarzian (see logo)
http://www.one-electron.com/Archives/Semi/SarkesTarzian/SarkesTarzian.htmlK200 silicon diode rated 260V RMS, 200mA.
See Radio-Electronics-1963-10, page 45. "capacitor and dielectric analyzer" project. (archive.org or world radio history sites)
https://worldradiohistory.com/Archive-Radio-Electronics/60s/1963/Radio-Electronics-1963-10.pdfIt is an early and vintage silicon rectifier, note that it is rated in AC V RMS like some vacuum rectifiers (5U4 rated so many volts RMS per plate, etc..) so 260V RMS would be 367PIV. LOL better to call it 260VAC!
sub given as 1N540 which is 280V RMS rating, 400V PIV, but 750mA which may be easier to find vintage. A metal-cased rectifier with a stud-type or 'top hat' appearance but wires instead of hardware, if you want to try to stay period as possible.