Southeast Asia in 1970 had lots of unofficial shotguns,and extra ammo was carried in a multitude of ways. Even saw some taped to the buttstock.
I've some 12 round pouches from those times, and recall some MTM belt carriers in OD that held 10 with a flip top. In civilian life later saw MTM with a 5 round version, never have seen the 10.
Also saw ChiCom SKS/AK bandolier vests with shotgun ammo, GIs are wonderful at adapting stuff.
As for extra ammo now, when I pick up my HD 870 there's 10 rounds on or in it. 5 rounds in a 6 shot mag, and 5 rounds in the Side Saddle. Had to crop one round's worth because of the peep sight.