I don’t remember the string phones as “send away for” items, but I built a few. Never a shortage of tin cans here….
(The family had connections with an outfit that distributed those vending machines that’d heat up a can of something and we got a case of little unlabeled cans once. “Mystery Meat” sort of thing. Since I was barely old enough to work the can opener, it was fun.)
I did send away for a submarine that was supposed to ascend and descend in the tub or whatever. Looked like fun from the TV commercial. It turned out to be about 2" long, and basically a solid piece of plastic with a hole in the center. Put in Baking Soda or Baking Powder (can’t recall which, but, naturally, we didn’t have any) and it did work.
Rather more of a rip-off, I think, than the two “space helmets” I ordered up, a few years apart. One turned out to be a military surplus gas mask, less canister and hose. Looked real enough if you could explain the big hole in the front over your mouth. The second was quite “true to life” v.s. the ones you saw on TV kiddie shows, but made out of styrene sheet (vacuum formed) that was about half the thickness required to survive for longer than an hour.
Method to their madness…. It got hot in there real fast, so having the front fall off….
The string phones always smelled funny, too…..