This is the second of at least 3 General Motors designs made in the late 1960s that feature a Lincoln head obverse. They were originally described as being made of 'powdered metal' but, according to Ken Potter, are actually made of bronze.
This variety is not listed in Pollock. It uses the same Lincoln obverse as P4055 combined with a different reverse die. We are calling it P4056. According to Ken Potter, die varieties exist.
Additional information on these pieces can be found in Pollock.
Photo courtesy of Ken Potter.