he is correct.. also their is a couple other extras in the kit.. Another thing you may find is a donut shaped ring, this goes in the FCG to hold everything in position. if you put it on the wrong side it will not hold properly.. It has been a wile so I am scratching my memory but if you realy get stumped I can get my Romak and give more detail..
If the pin you have pictured is about 1/16" diameter, then it is for the bolt hold open that keeps the bolt back when the magazine is empty. There will also be a very small spring, and the actual piece that catches the bolt. They all fit into the center support just behind the magazine well inside the receiver. If this sounds like your part, I can put up a couple of pics that might help you see where it goes.
I realy think you will find it is a bolt pin and thair is allready ONE in the bolt. JUST a spare.. the notch missing on the end is their to fit around and retain another part..look carefully at your bolt and the retaining pins in it you should find one in their ..