>If the "tickmarks" aren't primes, what are they?
vertical typewriter quotes.
>Are they simply
>surrogates with no identity/meaning of their own?
Well, they've meaning now in programming contexts.
FWIW, if you're using a font / platform combination
which doesn't have its own primes, these are available
in the Symbol font. On a Mac it's <ALT> 4 for a single,
<ALT> comma for double.