[700rpm]

I have (too often) punctured an innertube with tire irons either during removal or remounting the tire. That may have happened with you too. I’ve never had original metal stems though, so I can’t speak to whether they are prone leakage.

I also check the innertube by pumping it up and exposing it enough to water that any small leak might hiss and bubble, before I reinstall. When I was 17 (1962) I worked in a service station that had a Model A pickup as a service vehicle, and was taught to mount half the tire first, then partially inflate the tube, position the stem, then finish the mounting. Partially inflating the tube reduces the risk of pinching and puncturing it with tire irons.