It is not supposed to have a screw. The flipper switch might not be adjusted properly and the "fix" was to lengthen the button in order to reach it.

Assuming that it is the correct switch in the first place:

http://www.pinballlife.com/index.php?p=product&id=1518

Now you have a pic to post on the the worst pinball hacks thred.

It's usually noted in the cabinet parts list or drawing in the manual.

Post some photos of the switches and the name of your game and someone would probably be able to tell.

From the online parts book, page 157:
http://www.planetarypinball.com/reference/partsmanuals/WMS_Parts_Red_16-9190-B/index.html#/157/zoomed

buttons: B-12273-6 (x2)
switches: SW-10A-48 and SW-1010A-13

Screen Shot 2016-09-26 at 5.33.24 PM (resized).png

I would replace that switch altogether. Very rusted and just looks like trouble.

Those contacts have been filed to death! They need to be replaced, also there are nuts under the screw heads as spacers that shouldn't be there. Look at the start button for an example of how it should be, if the screws are to long you add a fiber spacer to the stack and if the switch is to far from the button you move a fiber spacer from cabinet side to the screw head side of the switch. Always replace both left and right switches at the same time.

Buy one for of these switches for the right side.

Williams/Bally Flipper Leaf Switch - Double Contact
Price: $8.95
Product ID : pbl_SW-1010A-13

Buy one of these switches for the left side.

Williams/Bally Flipper Leaf Switch - Single Contact Price: $4.95
Product ID : pbl_SW-10A-48

Tighten the flipper contact stack, replace the flipper buttons, and crank the pal nut clockwise with a set of bent needle nose pliers to get a snug fit.