Marc,
The 1954/55 clock (986633) and housing (986960) were Chevrolet accessories. This windup clock is the same part number for 1953/54 Chevrolet automobiles (the auto also had an electric clock accessory). GMC trucks had a different housing and I think a different clock.
The clock has to be wound up each day but it keeps good time. I have read on another website that the car and truck windup clocks had different location for the wind-up knob on the face. This seems unlikely, since they had the same part number (the dealer would order the clock itself from the auto parts book). Most likely, the manufacturer of the clock changed the location in different production runs.
I was lucky enough to find the NOS housing (in the box with instructions) and a used clock in Hemmings Motor News in 1996/1997. I talked myself into buying it - it seemed expensive at the time ($35). The housings have become much more expensive (over $150 on eBay) but used auto clocks can be bought on eBay for $15 upwards, depending on condition.
Tim
