There's another way to recognize the right end of the spring because the difference in size is minor. One end is wide open and the other has at least a whole turn tightly packed. The closed end goes to the back of the barrel.
The same rule applies to other guns, not only blowbacks. I've seen a lot of 1911 with the spring mounted backwards. If one end goes into a guide (spring guide or the barrel itself), it's the closed end. Ends that go inside a cap, the spring plug on a 1911 or the inside of the slide on a blowback, are the open ones.