Polymer 80 released their new compact longslide model a few months ago. Combining the slide length and sight radius of a Glock 17 with the compact grip and concealability of a Glock 19. However, they changed one part on these frames that causes a lot of questions from our customers. The Slide Lock and spring slot does not look like a standard Glock and does not take the same spring. Here is a comparison of the two frames.
PF940CL (Left) vs PF940C (Right)
Long Slide on left. Compact on right.

The Compact Longslide however takes a round spring that sits completely underneath the slide lock. This spring is included in the Polymer 80 Frame Parts kit included with the frame. 
Insert this spring into the hole in the middle of the slide lock lever slot and insert the slide lock on top of it.

I hope this clears up some confusion on the matter. If you appreciate the information please consider shopping with us. You can see our full line of Polymer 80 frames here