The most common 11 perf stamp is the flat plate version which is less than $2 mint.
The expensive 11 perf stamps are rotary printed which changes the length of one side
of the stamp. 594 and 596 are both perf 11, but 594 is "stretched" horizontally (top
and bottom edge) and 596 is "stretched" vertically ( sides). The difference is a
fraction of a mm ( 1/2 mm) in case of the 594.
I would have said that they were talking about #578 which is rotary press also and
perf 11 x 10, ( 11 horizontally and 10 vertically) , but it is only worth $160 used
which is more than the $85 mint, but not close to the thousands they mention.
578 is available on eBay
with buy it now @ 40 for a nice one. Hope this helps and doesn't
confuse. Basically, either their perf description is fuzzy or their pricing.