Anyone Know What This Means In Brunell?

Fort Smith, Arkansas:

1917: 1, 1dbl, 2, 3, 4, 4inv, 5, 5inv, 6, 6dbl, 6trpl, 7, 8,
9, 9inv, 10, 10inv, 10dbl, 11, 11inv,
11(p10x11x11x11), 12, 12inv, 12dbl, 13, 15, 20,
30.

Does "11 (p10x11x11x11)" mean that there are precanceled versions of the stamp with p10 at the bottom?

And is a regular collector ever likely to see one?

Thanks.

Not being a US collector as such, I would presume that the perf 10 would be at the top as that would normally be the side which is mentioned first. EG p15x14 has perf 15 at the top and bottom and 14 at the sides.
Couldn't tell you about your chances of coming across one, I'll leave that one to the experts!

I expect you are right about the location - I just checked the catalog and the P10 can appear at the top OR the bottom, but since Brunell put it first, probably in precancel form only the top is known.

FYI, my 2005 catalog says $5000 for a mint UNprecanceled example - but sometimes there are precanceled versions of quite expensive stamps that are common enough to trade for a relative pittance, so I'm curious about this one.

Yes - this refers to the 5000$ error. It does not trade at a pittance.