The W123 has a funny system. The front wheels look like this =>
or a bit like this without the vented disc
(Both pictures borrowed from this thread
Upgrading w123 with 1st generation w126 brakes)
The disc brake is bolted to the inner side of the hub.
On the back, however, the discs are more conventionally bolted to the outer side of the hub so for the backs you need a bigger diameter than the value I've posted.
If nobody else can help I'll dig about in my garage tomorrow and find the front hub diameter. If you use that value then you'll always be able to fit your wheels to the front and the back hubs