I've never quite understood the Astra panel. I think it was just meant to be a steady red lighted panel to the front (Hmm, would that have met the CA steady-burn red requirement?) and/or a fancy front lighted sign. I think your second picture, Bruce, says it best - there were a lot brighter lights on an S&S ambulance that would have washed out the Astra panel.
I'm with Steve that, probably, someone somewhere made it flash just to make it flash, and may even have ordered it from the factory that way. But from the ads and such, I don't think it was intended to be a warning light. I'm not one who has seen a ton of S&S ambulances, but the ones I have seen, it didn't flash.