Used for many years in Los Angeles County, by numerous Southern Cal. agencies... Commonly known as the "Platter". Paramount was--may still be! a contract City for the Los Angeles County Sheriff Department.
No they were not "shop made." They were a commercially available product through Trio Sales. Trio made them, but the originals were made first by a company called Coast Lighting. I believe Trio bought them out. Trio also made the T2 Can lights on LAPD cars. This was after S&M stopped. See the brochure cover above... The sirens varied, with Federal Signal and B&M SuperChiefs.