Any question answered...

Would this be the culprit?
It fits the braze on on an old Raleigh Twenty I have and it's definitely not metric. Yours if you want it. Free gratis.

As per @mdcc_tester 's reply I used a hard plastic bottle top and the neck it screwed onto to produce said bucket and drilled a hole to mount it with the bolt and a suitable washer. It was a bit of a bodge until I found something suitable or got someone to turn one out of aluminium. Neither happened but the bodge worked well enough.