Havn't seen one of these before!

[philward]

Not sure how this works - were they on sale in UK?
https://www.ebay.co.uk/itm/NOS-Stop-Lock-Honda-Cb750-CB550-Rear-Wheel-Permanent-Lock-CB-750-SOHC/202560707013?hash=item2f298f19c5:g:5AUAAOSwk3BbaNi3:rk:1:pf:1&frcectupt=true

[SumpMagnet]

Looks almost like that arm replaces the brake arm on the rear hub....then you apply the brake....and lock it in place through the extra hole in the arm.

That would be my guess.

[philward]

You'd have to be careful with your balance rolling the bike of the centre stand!