@floobydust, I'm pretty sure it's selenium (please see below). Vf is 0.4 - 0.46v depending on which diode is measured.
@Andy, I wish I had a curve tracer --- maybe there's an app for my scope or DMM...
@Terry, thanks for the link!
At the bottom of the pile, I found the top of the box, so that's a win-win! It's an RCA Full Wave-Bridge (Selenium), part #59504. Googling the part number, I found it being used in a broadcast audio Limiting Amplifier (RCA BA-6A), which makes sense considering the basement I found this stuff in.
https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/RCA-BA-6A-Compressor-2.pdfPage 13 of the manual shows it as "Rectifier, selenium, bridge, input 18 v ac, output 12.5 v dc at .780 amps", with an RCA part number of 59504.
...so I think I've got it. Thanks for all of the replies and links!
-Frank