Oh dear, thats a bummer !
I leave it to Alan for a final verdict

The combi of the plunger springing and the direction the cam on the lever pushes the shoes makes for a self servo action when the rear brake is applied.
A factory mistake !
Routing the rod with the arm pointing up is a bit difficult, some models used a cable after this was found out and you need ...

Hello Alan,
I'll take it with me to the rally anyway.
I am sure I can find someone that can take it with him, that way it will be in the UK.
That saves you importing it from the EU > UK.
If all failes I think of another way

Alan, I've send you a PM also.

I have one nos, and I can take it to the Dutch rally.
A few UK members will be there and I can burden one of them with it
But I dont know if it will be a member close to you...
Anyway, it's for free as I will not use it, my Ariels being either 19"or 21"

slowly I am getting there....
numerous small bits ready for primer

is it kissing the case all the way or in a specific spot?
If it is the point Roger is referring to you might have dropped the washer that was behind the inner case?
I mean the one right below the clutch dome tying front and inner chaincase together and in the same straight as the lower gearbox bolt.

Indeed, thats from another model.
Thanks for looking!

Thanks Bob !

I'll try again;

Wanted: BSA Bantam toolbox lid with one screw.
Just the lid I have the box, (repro ones dont fit a original box)
-for my HT-

Thanks!

Ah yes, torque arm, I could not come up with the correct word!
thanks Roger