Well this is from 1955, so the mis-use has been around for a while.
I think as Haz said, it was the art department who did it. The word "clip" isn't found inside in the instructions, which would have been written by someone more technical.
I don't think anyone would refer to the M77 magazine and say, "clip it in". "Slide" is more like it. There's a tab on the back of the mag that has to line up with a groove in front of the trigger guard and you slide it in place. Had this model caught on, who knows, the slang for magazine might well have been "slide".
In case you're wondering why the M77 didn't become the 10/22 of it's day? Plastic. The gun was easy to disassemble, including removing the barrel. Innovative left hand bolt. Good heft and feel. Adjustable trigger. But the whole trigger guard was one piece plastic and was looked up with disdain by the gun world in 1955.