Rolleiflex identification?

Was hoping the group could help me out. I purchased this rolleiflex yesterday from what I know it's a 2.8 E. What throws me off is the small screw on the side next to the film advance crank. If you press on it, it resets the counter. Has anyone ever run into one of these and when would one use the side button to reset the counter. For those wondering the counter resets itself as normal when you open and close the back. Thanks in advance.

This is a Flex 2,8E with meter. The type was produces between 1956 and 1959. The serials run form 1.621.000 to 1.665.999, so you have late type. You need accessories with bay 3 for that camera.

See here for dating that camera: Dead Link Removed

I've got one of these but not with that counter reset. Looks like someone had it modified to also work with 220, like what you see offered here: http://www.rolleirepairs.com/220mod.html

Ok the 220 modification makes sense! Thanks!