In a bit old Japanese, the phrase:
means "if you (talking to a superior) put on the straw sandals" (I assume from the context).
Where does the conjugation ～なば of 召される come from?
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This な should be 未然形 of ぬ.
なば is treated as a compound word in the linked dictionary page, and means "having done (something) ~".
I'm no good at old Japanese, but I think it says that when (someone) has put on (grass/straw) sandals, in polite language.