That's the one.

As far as "missing physics" goes, I still feel that it's a tad hubristic to assert that we've got everything nailed down just because we can measure electron mass very precisely. There could always be unknown unknowns, phenomena which we haven't seen before because we haven't observed the conditions under which they would arise. There could simply be regularities in our observations which we don't detect, like how both Newton's laws and relativity are obvious-in-hindsight but required genius intellects to be first observed.