I have had my 57 T-Bird since March and am still learning about it. I was looking at the driver's side of the engine, 312, and noticed that the oil pressure sending unit had been modified. Does anyone know what the big cylinder is for?. I can see the sending unit, but have no idea that the other part is for. Any insights?
.

Probably for Aftermarket Oil Pressured Gauge. Do You have an under
dash Oil Pressure Gauge.

  

The larger canister is an aftermarket electric oil pressure gauge sending unit.  The small unit with the wire attached is an old style brake light switch.  It most likely leads to a low pressure warning light. 

... old style brake light switch ...

Oil Pressure Warning Lamp Sending Unit (BPN 9278)

____________________________

I’ll go either electric oil pressure. We just installed electrical oil and water on my sons 40 to keep the bourbon tube and oil line from inside the car. Yours is an early sender and since there is no wire on it I’m sure it’s been removed. I see the idiot light sender is on the other end of the tee.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Thank you all for the info.  There is no under dash gauge, but there are some holes so there very well could have been sometime in the past.   There are other unexplained holes in the firewall, about 1.5 inches in diameter that are now plugged with rubber plugs. They are by the heater.  These look about right for two hoses.  Perhaps an added on air conditioning at some point.   Thanks for the info on the oil sending unit.  

If You want to remove the "T" Setup the Oil Pressure sender for Idiot Light will screw directly into the Block. Same Tread Size as the "adapter".