I've been up to Malvern again this weekend and found no less than two "holy grails".
The small pre war BSA adjustable is quite adorable, not very cheap but much more reasonable than the few I have seen before.
The two circular brass items are base plugs from WW1 Mills grenades, The Calthorpe I bought at the W&P show years ago, A dealer from Belgium came up with the James this weekend after nearly two years of asking, as far as I know these are the only two Motorcycle firms that made them but I would love to be proved wrong and find a BSA Rob
Talking of Motorcycle/grenade base makers, I've got James and Calthorpe ones like yours and I also have a Burman one stamped Burman and Sons. I did some research on this a few years back and found that Bowden Wires made them, stamped BW Ltd. There are two James ones, stamped either James Cycle or JCCo. There are three different stamps for Calthorpe being Calthorpe, Calthorpe Motors Co and CM Co. There was also Morris Motors Ltd stamped MM Ltd and Minerva Motors Ltd stamped MML Ltd. There were also bases made by C.A. Vandervell and Co stamped with their well know initials C.A.V. A little off track there were grenade bases stamped Q which were made by Qualcast.