This plate was originally issued in 1968 to commemorate the 50th anniversary of the Chevrolet Truck.
It was a small way for Chevrolet to express their thanks to the Indianapolis plant,as well as pat
themselves on the back for 50 years of production.
They were not related or meant for the pacecars in anyway.
They can still be found,mostly by technicians and employees who hoarded them back then,as they were given out free.
I think they were also offered over chevy parts counters then,and they were being reproduced not too long ago too.
The one below is a Repop.
