You could machine up a batch of heads and braze them on to threaded rod.
Or you could try knocking up a head on plain bar. Given the quantity of rivets involved in this forum, it is not a great difference.
Martin

Thanks Hedd, I saw this advert a while ago but it seems only three of the bolts pictured have the 'fat' or bulbous heads I would prefer. As you imply it would be expensive..It may be that I need to accept a lower standard of authenticity or make my own.

Thanks Martin. You are right, it should be possible to make up a female former into which a length of 1/2 bar, with the end suitably heated, was inserted and then press the top with a rivet snap, so forming the round head and the squared shank below it.
I am however in contact with a TT member who apparently has a stock of suitable coach bolts. I am awaiting details.

Either way the help offered by the respondents to my query is very rewarding and an excellent example of the goodwill in the road steam movement. Thank you all.