OT Signaltone air horn

Hi all, found this air horn at the tip just recently. Looks good, no rust or corrosion, just a piece of the mounting base has come off. All the insides look alright. Just wandering how to get this thing going? What operating pressure would it require? How old is it? It's made in America, as it says Livonia, Michigan, USA under the base. Any other specs would be appreciated.

Thanks.