What is the function of the pin with the gas port hole in it? The one with the spring that goes thru the gas block. I have never tried to firing it  in the other position. Would the other position make it a single shot? It looks like turning it 1/4 turn would shut down the gas system. just starting to learn about the M1A .

It sounds like your talking about the spindle valve. Yes it does shut off the gas flow to the piston which moves the action making it a semi auto and full on usgi models. Turning it will make your rifle basically a single shot. It was used when shooting grenades from the barrel as they mounted to the front of the barrel just like Garand's.

Here's one of the best sources to learn more about the M14 put together into a book also by Different a cherished member here and on other forums.

Different's M14 page

The spindle valve is also shut when using the line throwing attachment.  On many match rifles, the spindle valve is fixed in place.