Never had reliable readings with my oil pai gauge so i ordered a new sending unit. Faria gauge and i ordered a faria sending unit. Swapped it out no problem. Havnt had a chance to run the boat yet so not sure if i fixed it or not. But my question is while swapping it out i found another sending unit just to rear of it. Its a different style sending unit (looks completly different). At first i thought it was an oil temp switch but i also have one on the oil filter that i believe is for oil temps? So im not real sure what the other one on the block is. Any ideas? Hopefully i replaced the correct one.

502 block if that matters.

That would be an oil pressure switch. Do you have a low oil pressure buzzer/light or possibly an electric fuel pump? They are used to control things of that nature. I'm sure someone here with a 502 can tell you the exact function.

Gotta be an oil pressure switch.
If it has 3 leads and you have an electric fuel pump it is there to shut off the pump if the engine stalls. That type cuts power when oil pressure is too low. Or it can be a switch for the audible warning system. That one has a single lead and trips the beeper when there's no oil psi. Some of that style have a time delay for quieting the beep during startup.

Yea im stumped. Guess im gonna have to trace the wires. No electrical fuel pump. I do have a buzzer/audible system when u turn the key. I didnt know that would be seperate from the gauge. Both the one i replaced and the mystery one only have one lead. Just a push on connector. Ill fire the boat up saturday before i head to lake and verify all is good. Suppossed to be stupid hot this weekend.

Pretty sure what you are talking about it is the oil pressure sender for the Merc audible alarm system.
The alarm system uses separate senders for low oil pressure and high water temp. They use tan wire with blue stripe.

Is this the one on the back by the bell housing behind the #7 cylinder?

Thanks griff thats the wire.