I figured someone with more knowledge than me would have answered this one by now. I have an R-10 model which may be completely different from what you have.

With mine, there is no button as such, but if I feel under the back of the extended sliding breech I can feel a protrusion an inch or so in. If I depress that protrusion I can slide the sliding breech right off...Geo

By the way, welcome on your 1st post!