The OVP is not a relay, but rather a multi-circuit, circuit breaker. At least that's my impression of it's purpose.
This device has received as much attention here as any item I can recall.
You mention a 420SEL, but do not mention the year. I do not know if this applies in your case, but 300SELs from 88-89 used a fuel pump relay that was near the battery. 90-91s had this relay incorporated into the MAS unit.
Your best bet now would be to hit the SEARCH facility where you'll find a wealth of knowledge.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.