There's a research project going on the Model 51 it's on the Remington Society of America. If you go there look under current research one of those guys can certainly help you or visit the forums on the site. I've never removed the catch, I hate taking those things apart they're a pain.
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To remove the magazine catch from a Remington Model 51 pistol, first field strip the gun and remove the slide. Locate the magazine catch and use a small punch or screwdriver to depress the spring-loaded detent. While holding the detent down, push the magazine catch out of its slot, and it should slide out easily.