Charles,
To answer your question what does it do, it along with the other two hold the front of the floorplate in place. Along with the two screws through the stock at the rear, they hold the gun together. As long as all others are tight it will not in my opinion hurt to continue shooting it.
But with that being said, the gun that has the same screw in place shows that someone has been inside of it and either did not tighten it correctly (enough) to get them all timed north/south or they have the screws in the wrong holes. I would have someone with a correctly sized screwdriver who knows what they are doing to check them and get them timed, that is if you don't know how to yourself. If two screws are short and there is a long one it is for the rear. The left side has a single slash cut into end.
If all three are the same length: Left screw (under the left barrel) has a single slash, Right screw has nothing, and back screw has a + cut into the end. I've never been into a repro but am just making the assumption that they are like the originals.
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