When the M56 gear was adopted by the US Army it was during a very transitional period. Not like the trans periods in the Army today, Heyooooo!! Anyways.. The ammo pouches were first produced to be able to fit magazines and ammo currently in use and for the assumed 7.62MM Nato rifle of whatever model soon to be adopted.
The Original pouch would hold 2 mags for the M14 or BAR, a bandoleer of block clips for the M1 Garand, the M1 carbine, 4 grenades ( 2 inside, 2 outside) and a few 40MM rounds. Truly universal.
Well, we all know what happened. The M16 went into use and while the pouches held 3 M16 mags, they set really far down inside. Guys ended up putting a sock or a bandage in the bottom, tying cords to the mags to help pull them out and all sorts of things.

The Army finally listened to the complaints I guess and got around to making a shorter pouch to fit the 20 round mags of the M16. Better late than never but not in time to really do much good. Not many people for the shorter pouches and it wasn’t long till the Army went to the new Nylon M-67 gear with its nylon version of the shorter pouches anyway.
This puts these smaller M56 pouches into a now rare collectors interest. Especially if they are as mint as the one above.