I'm doing 2 71 heater boxes at the moment also ..
i just tore 3 heater boxes down,1 now for parts
and haven't had the chance to blast a few things
before i start to put them back together.
so i haven't really checked where all the seals go yet.

when i took my strato box apart it was filled up with acorns
and the heater motor wheel had a big chunk chewed out of it by some damn rodents..
i'm so glad i did not sell off a spare heater box i have
that is now being raped for parts.
i look at the extra kit i have and it appears that seal is in there.
i will find out when i cross that bridge.