1969 T120 650 Centre Cylinder Head Bolt Thread Type

Hi all, a mate of mine has the above and has a damaged bolt but is not sure of the thread type to get a replacement. Can anyone help with that..?
Cheers

 

Thanks for that,Speeder, much appreciated. We knew they weren’t metric but weren’t sure if they were UNC or something else as my mate thought that Triumph had made a change to the gauge they used. I think he may have it sorted now.