Good day.
Show a photo of the bracelet and sel from all sides.
SEL bracelet for 1950 case should fit snugly to the end of the case between the lugs, and due to this grip, do not scroll.
If it scrolls, then sel does not fit, or rather, does not have an anchor point at the bottom.
Considering that the font attachment point is a constant, sel cannot be moved closer to the case, nor can we add material.
This is a bad scenario, to predict the best - you need to look at the end links from all sides.
Also, if it is a good bracelet, then the end links should be separate and secured to the bracelet by watch rods that go through the holes in the lugs. In this case, you can choose a more suitable SEL.
In other situations, most likely you just bought a bad bracelet, or not intended for this model.