A few months back, I decided to take out and grease the small bolts holding the seat stay to the rear dropout on my HSG. Unfortunately both sheared off as I was unscrewing them. For some reason they were aluminium, and appeared to have been custom made...
After a lot of searching for bolts that would work, I eventually found these (M10, 1mm thread pitch, 14 mm head diameter):
https://www.accu.co.uk/flat-head-soc...-HPPF-M10-1-A4
Unfortunately the head diameter is not actually 14mm but 16 mm, so I had to put the bolt on the end of a an allen key in a drill, and run the edge of the head against a file to reduce it down to 14mm. Hallelujah, they work!
Thought I would share here in case any one else hits the same issue.