That looks like a knockoff of the common Saunders easel - the knobs move the blades up and down/back and forth, so it's easy to precisely frame up a given size.
I'm basing the following comment on my Saunders 16x20 easel, but... I can print 20x24 on that easel, as long as I don't mind borders of about 3/4" (And I like a good sized border on a large print). You just stick the paper in without pushing the edge into a slot. So for a slight compromise, I have a solid easel for all standard print sizes, that's not massive and was much more affordable (and cheaper to ship) than a 20x24. (Sure, I'd love to have a 20x24, just don't want to pay for one!)