The plastic bobbins were manufactured and sold as replacements for the class 66 bobbins in the 60's.
Clearly the Singer Company thought they would be all right.
And they're still being sold as class 66 bobbins.
If there were something wrong with using them on a machine made for class 66 bobbins, wouldn't Singer have re-classed the plastic ones or maybe put a disclaimer on their current packaging by now? "Not to be used in machines made prior to 1960" or something like that?