I collect PAM and Telechron 15" beer advertising clocks. This style of clock consisted of a bubble glass front and then another layer under it and this was an advertising glass. The clocks came standard with two 15 watt light bulbs for lighting up the Ad glass
PAM clocks had aluminum cans (the back cover basically). Telechron clocks used a compressed fiberboard material for their backs. The Telechron backs were not built for the long term, and it seems many of the fiberboard backs you see today are in rough shape.
Another area that caused issues in both companies' clocks was that people would use 40 watt, 60 watt or whatever wattage bulb they had to light the clocks up. This caused many. many of the clocks to have their Ad glass burnt, and the excessive heat caused peeling paint. Thus, they are difficult clocks to find today in really clean condition.
Explore the PAM and Telechron links above to see some of the clocks I have hanging up.