Think I have it figured out. I stumbled across this 1981 RCA cross reference, and it has the MF12/1b part number from the original rectifier. This crosses to an NTE 502. I found one on eBay and ordered that. Should be just a hair longer than the original, but otherwise the same.
Will post back when I get the new part installed.
Cross reference found here:
http://bitsavers.trailing-edge.com/c...Guide_1981.pdf
