When is F3 Not F3 in MLB?

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I read a situation where the umpire crew in a Cubs/Pirates game made Cubs F3 and F4 switch gloves due to the defensive alignment.

R1 on 1st. F3 plays way in for the bunt. F4 comes over to hold runner close to the base. Hurdle brought it up and the crew chief agreed that Rizzo and F4 had to switch gloves since MLB rules say that only the first baseman can wear a "mitt."

Maddon says he has been given different interps from different crews.

What is there in the rulebook or umpire manual/interps for MLB that supports this?