The Model 50 is an open bolt .22 and Marlin's first attempt at a semi auto .22. It's also the only mag fed Marlin you can't find a replacement mag for.
It was only made a couple of years, and the early model like mine had no extractor, the next version had a manual extractor on the front right side of the receiver.
The A1 versions were all closed bolt. Here's some pics of a Model 50 open bolt: