There's more info in the VIN...
The first digit should be "1" for Plymouth Valiant, 6cyl
Second is the trim level. 1 would be Valiant V100, 3 is V200, 4 is Signet
Third is the model year. 4 for 1964
Third is the manufacturing plant:
1 - Lynch Road, MI.
2 - Hamtramck, MI.
3 -...