
Solved it...?

I think I have it's most probably a;
- 2" x 3/8" 16tpi BSW Whitworth (14mm) Hex Bolt BZP Full Thread
close enough to M10 x 1.5 to Tap it out... as that's a bit of a hard find, you'll get one in a big multi pack of bolts maybe.

I'll post an article up here once I've done my 10 newbie posts, until then...
BMX Handlebar Stems are a weird one too being M7 with a Slim 6mm Allen Cap Head and often integrated washer and unique lengths.
Just sayin' as I've also got some 10.5mm x 26tpi axle nuts, along with other strange bits.
I collect them...