it is a M5 which means it is a 5 mm metric thread your Industrial Hardware store may have them in various lengths otherwise try your local boating chandler that or any good sail boat supply shop, hope that helps if worst comes to worst I'll pick one up here and send it to you but I am not sure they make them in 60mm long and also what sort of head do you want hex key cup heads are generally most likely to come in that length.
I am about to go down there (my local industrial hardware store) in the next hour I will report back as to whether they make them that long
