We have a similar problem here in England.
Around 1970 the electrical industry switched to metric, adopting 3.5 mm threads on device boxes. Prior to that, device fixing screws were 4BA (BA = British Association).
The diameter and threads look pretty much the same at first glance, but are not interchangeable. As there are still plenty of 1950s/60s boxes in use, it's annoying to find the screws supplied with a modern outlet or switch don't fit the old boxes.
