Top one looks to be your gauge sender, as in, a variable resistance device.
Bottom unit certainly looks like a pressure switch although I can't make out the brand. Three terminals indicates it most likely has a common terminal and both normally open and normally closed terminals. If it were operating a low oil pressure warning alarm/light you'd be using the common and n/c terminals.
If you want to confirm you can check them with a multimeter. If you're not sure how I can give you a run down.
You're not looking at a small block though. Looks like a 351C or possibly a 400. These have two oil pressure ports, one front where the Windsors have it and at the top of the block behind the intake
The rear is definately a stock style electric oil pressure gauge or even idiot light.
The forward is an aftermarket pressure monitor for a kill switch, or more likely an electric fuel pump.
The best way to run an electric fuel pump is to use a switch like this that doesn't start the pump until oil flows. Before oil pressure, the engine runs for a second purely off the fuel bowls. This keeps you from needing a manual switch, or having the fuel pump run anytime the key is on.