Technical Mixing backing plates?

On a juice brake conversion for a model a, why in Les Andrews book does he say to not use mismatched backing plates on the car and why can’t you use 39-41 backing plates with 46-48 drums and vice versa?

RE: backing plates - what’s the issue if you use early plates on the front and later in the rear?

In both sections, it’s doesn’t explain WHY you cannot mix parts. Why can’t you mix backing plates front and rear and what’s the issue with drum and backing plate compatibility?

 

Maybe he was referring to side to side, not front to back.

 

Thanks. That’s my****umption, fronts and fears can be diff year.

still curious why the drum and backing plate have to match year. Both drums have an I.D. of 12”. ****ume the braking surface is wider one year to the other?

 

I’ve run one 40 drum on the left front and one 47 drum on the the right front with no issue.

 

Thank you