Build Date question

[CrispyK74]

Hey all,
I am trying to find out my 71 challengers build date. I have the build sheet, but I don't believe it tells me when it was built, the door tag is gone and so is the fender tag. I know that the car was built in the Hamtramck, MI plant and I believe it is the 331776th vehicle. Please help if you know. If you need more information please let me know.

 

[DetMatt1]

Assuming the front crossmember is original to the car, there's a date code stamped on the forward facing surface directly under the engine.