Replacing Auto-Lamp 575- what's this second bulb for?

[Globie64]

I'm replacing the Auto-lamp tail-lights on my 64 Globetrotter, and I got some stop/turn/tail schoolbus fixtures from etrailer.com. They came quickly and look good, but when I took off the old wedding cake lens, there are two lamps and three wires. I don't have the tow vehicle handy to light it all up, but I'm wondering what the second lamp is for. I thought it was going to be a single bulb with two filaments, stop and tail, and the stop lamp flashes for the turn signal; this is what the new tail lamps have.

Thanks

[Globie64]

OK, well I think I've answered my own question: After looking at a wiring diagram online for my year trailer, it's clear what the other lamp is for. The turn indicator has it's own separate filament. I can add another socket to the new back plate and make the new fixtures work the same as the existing, but with nice new foam backing gaskets replacing glooped on caulk. And, I can retrofit AL 575 lenses when I find more

[Rambler 64]

Hi Globie64,
I am replacing the same kind of lights (auto lamp 575 wedding cake style) and can't seem to find any replacement lenses. I need four of them. Do you know if the new 7" lenses from etrailer.com (A90RB) will fit over the old 575 back plate. Thanks Jim