Hmmm... I can't see how those holes would be related to leg straps. I've certainly never seen pics of any soldier with straps looped through them.
Yes, they are mag pouches within the leg pockets, as there are in the chest pockets. There was a uniform design trend at the time for this sort of thing. Obviously it proved impractical so it was dropped.
My guess is these interior mag pockets were designed for extras with the primary 4 still going into mag pouches on a belt.