Some of my old machines don't seem to care what kind of bobbins I put into them.
I can even use the very cheap pre-wound plastic bobbins in my 15's - though the machines do seem like the heavy solid metal bobbins best of all.
When the plastic bobbins came out in the 60's, they were sold as replacements for the metal ones. I used them in my grandmother's 401 and they worked just fine, so I never gave it another thought.
Since I started collecting vintage machines a few years ago, I've had a couple of slants that didn't like the plastic bobbins, but the rest haven't seemed to know the difference.
I wouldn't try the reverse, though - I wouldn't use a metal bobbin in a modern machine designed for plastic bobbins.