Thanks for sharing. I'd like the Kona eagle eyed to add their views. From the picture it looks more 8 than 9. Could you go old skool and try a brass rubbing, or something like that to check the marks?
At #263, with solid triangle drop-outs and a complete canti bridge and noodle, it's more 1992 than 1991 as #263 would place it as the earliest 1992 on record so far. The highest 1991 is #081, and the lowest 1992 #281.
Back to 18 or 19, we have no 18"s on file until 1993 which is where I'd love to call on Pip and clarify if 18" was an option between '90 and '92 and whether the lack of them was to do with the measuring format during those years or something else.
To add to this clarity conundrum, we already have B18263 on the list, as a 1994 model, with all the 1994 identifiers! We do know sequential numbers for these frames sometimes over-lapped, but still, it adds to the story.