Not sure what you mean by 'gearbox pivot stud' for a swing-arm Ariel, the part number 42-4077 seems to be for a Triumph part.
Ariels have a long through-bolt that screws into the frame, with a smaller diameter extended 'nose' that is used for the lower chaincase mounting (with a 5/16x26 nut).
This page on Drags' web site might help (hopefully!).https://www.draganfly.co.uk/index.php/c ... duct/4201-
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)