Great news! A nearby building that’s being renovated had a dumpster by it. Seeing some conduit, I decided to go digging and was able to find a 2903 strobe circuit and a 2901-9838 horn. To my surprise, both of them worked!
But I’m still missing the plate, so in the meantime I put them in my 2903 light plate. But I have to ask: Does anyone have a spare 2903 shell they’d be willing to sell me?