The specialist for your oldtimer.
en.hoffmann-speedster.com
...I haven't bought anything from them but have bought bits of werk34.de who probably also stock them.
If you have a bit of the right sized 3mm plate or whatever, you can make up your own - drill a hole for 'the middle of the fingers' then cut up to that, drill n tap the hinge captive screw holes...
If you don't have a tap set, welding nuts onto the rear would do but you'll reduce the 'hinge plate' adjustment as the nuts will hinder the plates range of 'float'