The second cap is the fuel sender,at the time the pic was taken
i was using the old cable operated fuel sender which has a nice cap on it.
i'm now using the electric sender,and with a little bending of the sender
arm,it now reads correctly with the late speedo/fuel gauge.
mine holds down via two bolts welded to the bottom of the tank
and pass through boot floor and bolt from inside the car,and a bar
welded to the back of the tank and again bolting through the boot
to stop the tank rocking.