Well done Richard! Bow to you!
I suspect you'll find other solutions pretty soon. I'm still holding my breath for a solution with shafts {1..21}.
Perhaps the original source was referring to that being unique. Or it has been proved utterly wrong with a brute force search like this and thus has disappeared from the googling range over these years.
To confess, I don't have the solution with me all this time.
(It was on a corrupted old hard drive which I still haven't found time to recover. But I definitely know a solution exists, which might or might not be the one Richard found first. From vague memory it doesn't look like it so I guess there are probably more. Luckily Richard has found at least one solution to bail me out, or I would have to spend a lot of time digging it from my corrupted drive or some old archives.
)
Also with that Richard has become the official winner this time (well he is no matter what). So whenever you like you could post another brainteaser/riddle. Hopefully it won't be a cpu-teaser (or more exact a cpu-workout) like mine.