The case should say the part number pretty clearly on the outside of both halves. The numbers will be something like 901-101-102.2R, 07R, etc.
According to my spec book, your first number 6135540 was:
6 = 6 cylinder
1 = engine type T
3 = model year '73
5540 = sequential motor.
The second engine number 6221267 would be:
6 = 6 cylinder
2 = engine type E
2 = model year '72 <<<
1267.
So you most likely have received an earlier Mag case which is not as good. This is especially considering that you had a 73 case which very well could have been one of the 2.4 liter "7R" cases which are very desirable because they were the strongest mag cases made.
It looks like you may not have made out too well...
