Hi Michael,
I have a 1968 250SL with the same setup as yours. From the firewall and forward: Relay for wipers; Relay for Starting solenoid on the back in injector pump; Relay for solenoid on cold starting valve (identical to the relay for the Starting solenoid -- both 4 prong relays).
Additionally, the 1968 USA models have a relay for the Fuel Shutoff solenoid and it is typically located behind the battery and above the coil. It appears to be the same as the other fuel related solenoids but may be a five prong relay for all I know.
On the right side fender well, near the expansion tank, is the Voltage Regulator (metallic finsh). Although larger than relays...it is often mistaken for a relay.
Best Regards,
J. P. Mose
1968 250SL