It is probably a selenium rectifier. Has metal fins?
Replace it with a silicon bridge.
The selenium emits poison when they fail...
Selenium rectifiers had a shorter lifespan than desired. During catastrophic failure they produced significant quantities of malodorous and highly toxic fumes that let the repair technician know what the problem was. By far the most common failure mode was a progressive increase in forward resistance, increasing forward voltage drop and reducing the rectifier's efficiency. During the 1960s they began to be superseded by
silicon rectifiers, which exhibited lower forward
voltage drop, lower cost, and higher reliability.
[6] They are still manufactured for exact replacement purposes, but are not designed into new equipment.