I've come a cross a passage in a novel to be translated, describing a
batch of hand-adressed envelopes. the sentence "excelsior, 500 count"
is included in the line with no explanation.
I thought at first it might be one of the old Imperial named sizes of
paper, but I can't find any mention of it.
Excelsior might be the name of a make of paper, rather than a
size. The 500 is presumably just the number of envelopes.
Can you give more context?