Backing paper? I've never heard of this problem. I developed the film in a rental darkroom and will go back today to see if the trashcan has not been emptied so I can look at the paper. Thanks for the suggestion.
You won't be able to tell by looking at the paper. The problems, referred to generically as wrapper offset, occur because of interaction between the very sensitive emulsion and the
front of the backing paper + ink + any humidity that might have migrated into the sandwich that the emulsion side of the film is pressed against.
The mottling is one way that the problem manifests itself, and has recently seemed to be affecting Ilford films a very slight bit more than usual (still very infrequently though). Kodak had different problems with backing paper a few years ago - wrapper offset that resulted in the numbers and words imprinted on the backing paper showing up as
increased density areas in the negatives - and it came close to ending all Kodak production of 120 film.
Wrapper offset has always been an issue with backing paper - since even before 120 film was introduced more than 100 years ago. It seems to have become more common since the films became more modern, and the paper sources have mostly disappeared. Backing paper itself is actually quite complex.
A few years ago one of the Harman/Ilford directors shared here that the cost for them to purchase the backing paper for one roll of 120 film was greater than it cost them to make the film for one roll of 120 film.