They are small lamps, bulbs, about the size of Christmas lights.
It takes longer to get the radio apart to get to them than it does to replace them.
The R-72 uses four total.
Order # - 5080000170
Description - Lamp HRS7219AY-230
Two dollars each, last time I ordered.
BTW, the IC-R7100 uses the same lamp.
When one goes out, I replace all four and save the good one for spares.