I can't say that I've ever observed that, but
then, I probably never
looked for it either. 80 bytes starting with HDRx, etc. is a pretty
good indicator of the nature of the block. I've seen lots of tapes
with 81-character records, however. (Univac 36 bit systems for
example).
Drat. I was hoping you'd have this answer. ;)
Looking for filemark-less transitions either way between 80-byte records
with known label names, and other kinds of records, is what my detector
script for this does.
De