You don't need 'some books' - as long as you buy a copy of this one:
'Packing Iron - Frontier Gun Leather of the Old West' - by Rattenbury.
Buy that one first.
The 'drop-loop' - also known as the 'Buscadero Rig' - is a 'Silver Screen Cowboy' affair.
Slim Jims, Mexican Loops, Cheyenne-style, Meanea or Miles City style, Half-Breed - Pouch and more - were all worn during the Frontier era.
Vaya,
Scouts Out!