Oh, so you can buy them on eBay. Of course.
Around here, they often replace individual lamps with LED modules. It's not uncommon to see a signal with red LEDs but incandescent for the green and amber.
What do you do with your collection?
Correct. When New York City changed incandecent bulbs in traffic lights for the LEDs, they were using these modules that replaced the lamp assembly. But the housings stayed the same.
The modules would get fitted whenever the traffic light required relamping. It's probably easier than having to take down that entire heavy monster and then mount a new one and resplice the wires.