I see this on a lot of old style duty belts, along with Lund's retro Highway Patrol Officer

 

Revolver rounds. Back before speed loaders were common issue each pouch held 6 loose rounds.

 

I think they can be used with speed loaders too, but I'm not sure.

10 minutes ago, c13 said:

Revolver rounds. Back before speed loaders were common issue each pouch held 6 loose rounds.

 

I think they can be used with speed loaders too, but I'm not sure.

Only for use with loose rounds. The speed loader pouches are massive cylinder shaped pouches.

They're dump or drop pouches by the looks of it, for carrying loose rounds as Lundy said.  Some had snaps on both the top and the bottom, so you could un-do the bottom and the rounds would fall right into your hands.

 

Speedloader pouches tend to be a bit wider and either cylindrical or boxier, like this;