Well, for anyone who may be interested here is my conclusion after the input from JulianS and Bess and my own follow up investigations.
My A10's gearbox abutment lug is the threaded type with a slot in the top for the cable inner to drop into (see photo) so cable pt no 42-8775 is not correct (as pointed out to me) for my gearbox inner cover, which was designed to accept the earlier cable with the adjuster at the gearbox end and not the handlebar lever end.
To change the cable for the earlier one, to suit my gearbox, I would need to change the handlebar clutch lever.
Not to end up with different style levers I would then need to change the front brake lever and then the cable for an earlier type with adjuster at the brake back plate end. Next thing to check was does my 8" front brake have the threaded type of fulcrum pin - answer is no (see photo) so that's not practical.
My conclusion or my guess is: Someone in the past has change the gearbox for the earlier type used prior to 1960 and that was why there was a bodge, with a washer, to be able to use a 42-8775 cable. My bodge was a refinement of that - and as was pointed out the cable didn't have the right shape nipple to fit the operating arm anyway. I think it might be a Triumph cable passed off as a replacement BSA 42-8775 cable.
My easiest and least expensive choices, it seems, are:-
Buy another 42-8775 cable (making sure it has a nice round nipple to fit the gearbox clutch operating arm socket) and after spending an hour filing the ferrule down to fit the gearbox hole, fit it and keep the one in use at present as a spare.
Or an alternative (as suggested by JulianS) drill out the hole bigger in the gearbox abutment to take the ferrule of the 42-8775 cable.