Rivco sliding door latch/lock

When the lock lever is moved to the full unlock position, the latch is retained and subsequent movement of the lever will not extend it. But when the lever is moved only far enough to unlock from the striker plate, then the lever will successfully complete the lock procedure by a full downward movement of the lever.

I've tried lube with WD-40 to no avail. Your product 82-218 looks like the latch in my Rivco slider, but I am not sure. What is the function of the slot head screw located just above the latch hook? Any advice would be greatly appreciated.

Bob