I got a Beretta 950 .22 short on a trade for some work and have a mag question.

On the left top side there is a cut out that runs down the side of mag body ,you can see the spring and follower thru it.There is a fixed screw that seems to be under the follower and above the spring......it is a flat head tip.
This screw is at the 6 - 12 : o' clock position,it can turn it to the left to 8 o clock and back to 12.While at 12 o clock I can turn the screw to the right to 1 o clock ( there is some kind of spring tension) and it will return to 12 p clock, when I do the 1 o clock thing the follower tips up with the turning of the screw and goes back down when I release the screw.

Here is the question.......what is the screw for?