If that be the case is there a device for reading fault codes on a pre OBD port?
There will be a diagnostic socket. It could be under the bonnet though. The OBD-II standard stipulates that the diagnostic connector should be somewhere around the steerng wheel. But before OBD-II, the diagnostic socket could be anywhere, sometimes took a while to find it too.
In the UK, the OBD standard was introduced/mandated from 2001. It just started to appear IIRC in late 2000. So for cars earlier than 2001 there were a number of tools around that often needed to be adapted to suit different sockets/pinouts and data transfer protocols.
For example some diagnostic tools connected using the K-line standard. I don't know if Mercedes used this method though. (I just can't remember, sorry).
I may be wrong, but it is unlikely IMO that your UK spec R170 will have an OBD port, though checking out the bonnet release area is a very useful suggestion from Todd157k.
DanSLK is probably the best person to know about the details. I don't have access to an R170, so can't have a snoop around.
You might be able to find some old diagnostics gear on e-bay. If you can find some of the old Snap-On diagnostics systems, that may get you there.
I'll have a dig about in some of the older databases and see what I can find out some more details for you.