b wiffled:
looked at a recently made (last 2 or 3 years i suppose) tdk d90 audio
cassette the other day and noticed there was no metal plate behind the
pressure pad - seems this omission is widespread these days- why has it
disappeared and what was its original function? something to do with
stray magnetic fields??
regards, b.

It's a good question, which has been answered, but another occurred to me:
how does a cassttte compare to an MP3?