With the serial number as posted, the gun would actually be a 1903 Lewis model. Forgive my non-technical jargon... but the button pushes up against an oddly shaped fulcrum backed by a flat spring. My guess is that the button just doesn't have enough clearance because of dirt/oil build-up. I may be able to post a picture later if it would help. You only need to pull the two interior screws and the metal should just pull out of the forearm, no need to fear an explosion of parts.