This is the key in question, before it lost a notch, and while it does work on most locks from the time, just looking at it tells you how well it’d work on a modern lock, and our front door had been modernized, unfortunately for my schemes.
Had a ton of fun with it.

It’s an issue for real keys, where someone can get a key cut to match a photo, but this is a skeleton key, it’s not keyed to anyone’s lock in particular and is freely available to purchase. Anyone with vintage locks that work with this key should just consider their door unlocked for security purposes, and either replace or supplement it with a modern lock.
So yeah if you want one you can get them at most hardware stores, it was like 6$, and came in a pack of two. (I lost one like a day after I got it, so I only ever really had the one).