Old Golden Guernsey Advertising Clock

This is old Golden Guernsey wall clock measuring 15 inches in diameter. The back can be removed & there are 2 small light bulb sockets. The inside works of clock are marked SYNCHRON. The back has old paper label reading NEON PRODUCTS, INC over LIMA - OHIO. My husband found in basement of old meat market in Milwaukee, WI about 35 years ago. It is metal - maybe porcelin (I don't know how to determine that). It is in good working order & does light when bulbs are in place. Golden Gurnsey is local dairy in Waukesha, WI. I'm interested in any info about clock especially actual age & how to determine if porcelin or whatkind of metal. Thanks for help.