As said, the OP is a Pattern 1876, Third Type, Prairie Belt with the 1879 Infantry Modification. Because the
Army was in such a hurry to finalize design and deploy this Belt, they apparently overlooked the fact that there was no adopted pattern holster in use with a loop large enough to slide over the Pattern 1876 Belt. Hence the various experimental modifications to the Belt through the early 1880's. These belts remained in use until the Mills Belts, including the Pattern 1885 Prairiie Beit, and the Pattern 1881, third type, Holster were deployed, which solved the problem.
Until then the Cavalry intended the Pattern 1876 Belt to be used with the Sabre Belts, holsters and 45 Colt ammo boxes/pouches in use during this period. Of course the troops did not want to wear two belt rigs if one would do, especially in the high temperature frontier. So many unofficial field modifications were made to these Pattern 1876 Belts (and holsters) to accommodate holsters and related accouterments. Thus unmolested examples of this belt are difficult to find today.
Pictured below is a Pattern 1876 Prairie Belt in unaltered condition shown with a Pattern 1872 Metcalf Belt rig, including a type II Pattern 1881 Holster, a Colt Factory restored Ainsworth Cavalry and period ammunition.