> These tapes have been generated with a simple tar command, e.g.
> tar cv ...
> so they are supposed to be 512-byte fixed blocks, right?

Most versions of tar generate 10240-byte blocks by default; that's
certainly been the historical standard.  (On nine-track tapes, which
AIUI were current when tar was originated, tape consumed was
(D*bytes)+(IRG*records), where D was something in the milliinch
range[%] and IRG was something like half an inch.  IRGs (inter-record
gaps) were to give the transport time to start and stop the tape
between records, I believe.  But as you can see from the formula,
512-byte records end up using tape very inefficiently; I assume that's
why 10k records were used.)

[%] It's been so long I'm not sure, but I think they were 800 bits/inch
(or bytes/inch, it's the same since the tape was written in nine
parallel tracks), later 1600 bpi, much later some much higher value I
forget.  Assuming 800bpi and half-inch IRGs, 512-byte records waste
nearly 44% of the tape in IRGs; 10k records waste only 3.76-%.

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