Hi Lynne,
the coachbolt is about 2 1/2 cm diameter,, just looks like a blob of glass, that the round top of
a bolt was pushed into, so its like a little bowl, it does leave a little molten glass 'scar'
in the middle, but no flame mark.
Hope this helps,
i will try to get a good photo of one when i get time.
Cheers Andy