22 tube magazines always have a loading port. You can see it in this pic, about halfway between the forearm and the muzzle. Shiny spot.
You pull the brass follower part-way out of the tube, so that the loading port is unblocked, and load through the port. I suppose this is to prevent you from accidentally laying your follower down in the dirt, or something like that.
With 22s that the magazine is in the buttstock, like yours or the Browning 22 automatic, it works the same way. You don't pull the brass follower all the way out. You just pull it out far enough to expose the loading port - that hole in the stock.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.