Crewe Engines are normally the recommended repairer of Mercedes-Benz engines - don't know if they also rebuild gearboxes ?
Might be worth asking - otherwise they might know .
I might still have a Fintail gearbox up the garden somewhere ( out of a W111 220b ) ,
Edit - just checked and it is still there - all gears were selectable and working before I broke the car . If it is of any use to you , you can have it for free - either collect if you're ever in Scotland , or organise a courier to uplift it ?
Dave Collins might also be able to help . A gearbox out of an early W123 might be the same too ?