The Federal model 17 rotating beacons came out in the 1947 time frame with patent in 1948. A lot of police cars either had red lenses over the spot lights or a large red lens lamp with a siren up on the roof. There were various light applications utilized before the rotating beacons became widely used by fire trucks and law enforcement vehicles.
Some smaller departments may have just had two blinking red lights mounted on the bumper valance or fenders.
Here is a good link.
http://www.lawenforcementservices.bi...26_Sirens.html