Webster's International Dictionary says the apostrophe may be used in "the plural of letters (as in two a's ) or of figures (as in three 7's) and sometimes in words that are normally nouns (as in no if's or but's), to set off an inflectional or derivational suffix from a word that is pronounced by uttering the name of each of its letters (as in their IQ's, he OK's it...)"
It follows that 60's can indeed mean the plural of 60. John Murray need no longer be "driven to distraction". It's simply a matter of him broadening his horizons.
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