How wide is the bushing in the drum?

The '71 and later drum has a wide bushing (3/4") and uses the narrow-lip seal. '70 and earlier drum has a narrow bushing (1/2") and uses the wide-lip seal.

If your drum has the narrow bushing and a narrow-lip seal, somebody installed the wrong seal. The later drum will accept only the narrow seal but the early drum will accept either one. The narrow seal in the early drum will work but it's not ideal.