[smfdco1]

The light you are looking for is a Federal Model 17. This was the first revolving emergency light and was introduced in 1948. It contained two revolving sealed beams. Later models carried 3 or 4 lamps. Since your truck is a 47 the fire apparatus part could have been built in 48 and thus had this "Beacon-Ray" light. They are around, were built in 6 volt or 12 volt and the chrome skirt was available for both flat surfaces or slanted for curved roofs.

I had 4 of them about 6 months ago but have sold all of them. You can check ebay but expect to pay about twice what they are worth. You should be able to find a nice one for under $200 somewhere. If you need a 6 volt expect to pay a little more.

If I can put my hands on one I'll pm you.... Good luck in your search