The VIN looks like a 68 unit for a Chevelle...
maybe the "8" is a "3"..??
the stamping looks like a 1973 date with M21 letter code..
1973 M21 was only available in Corvette, Chevelle and Camaro..
the letter code designation began in the fall of 68 for the 69 model year...
so...perhaps a closer look at the numbers to clarify the 73
-wilma