Dickinson College's digitization of the grammar text by Goodell [seems to suggest](https://dcc.dickinson.edu/grammar/goodell/verbal-nouns) that -τρον and -θρον are synonyms. We also have πτολίεθρον, where it looks to me like the suffix is -εθρον (unless this is somehow based on one of the plural forms of πτόλις that has an ε). Is the choice of suffix explainable based on some phonetic rule, possibly related to ease of articulation or historical evolution? I would have found ἄρτρον and πτολίθρον just as easy to pronounce as the actual forms.

It's cool to imagine that in some parallel universe, hydrogen atoms have electhrons in them.