[Ron Ramirez]

Thanks for that link, Bob - now I know that "intermediate base" is equivalent to the modern terminology "E17 base".

The correct bulb itself is either T7 or T8 size (tubular).

I think if I had a 15X or 15DX now, I would use an LED like this one:
https://www.1000bulbs.com/product/209031...etail-tabs

Advantage: Less energy usage, and you would likely never have to replace it unless you used the radio several hours a day, every day.