Sherwood,
Good question. Reference USPSA Rule 5.2.4 Unless I am wrong, after the start signal you retrieve your mags from the can, they have to be placed in mag holders according to the rules that apply to that division. Something good for the CRO and the ROs working this stage to maintain consistency, watch for and explain during this match.
Course designer for stage 1 & 3,
Jeff LaFlame
Mags can be carried in the hand.
Paul Hernandez
2013 Ohio State Championship Range Master