The "modern" twistgrip came in during '38 or '39 (I think '39) with the last version of girder forks. These had check springs and 3/4" bars that were mounted without the rubber handlebar isolators. I have a '46 VB with this setup.
I think competition VH models had the straight pull throttle a couple of years earlier (mid '30's).
I can measure a '34 twistgrip (this is the same as the '37 version) if anyone needs to know a length. I can't help for earlier than that.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.