I'll take a wild stab at it.
All the way thru '65, the front drum was P/N 113 405 615A
and it used tiny ball bearings.
Beginning '66, the wheel bearings (and shoes) were changed
and the drum bores revised. The new P/N is 131 405 615A
If not careful, the numbers can easily be transposed and the
computer will ship the wrong part.

All the way thru '65, the front drum was P/N 113 405 615A
and it used tiny ball bearings.
Beginning '66, the wheel bearings (and shoes) were changed
and the drum bores revised. The new P/N is 131 405 615A
If not careful, the numbers can easily be transposed and the
computer will ship the wrong part.