Western saddles are an example of an item that varied a great deal based on decade and region. Rigging style, cantle height, pommel style, fender shape and square versus rounded skirt were all things that varied by time period and geographic location. A saddle made by a saddle maker in the northern plains for example, is easy to distinguish from one made in the same time period from a saddle maker in the southern plains.
My impression is that holsters also varied to a great extent like saddles, with traits that are often associated with specific time frames or regions.
However, to somewhat over simplify it, it basically boiled down to a few general concepts:
1. A high arched mouth that arched up to cover the cylinder and recoil shield, and then came back down to either expose most of the the trigger guard
2. An arched mouth that covered the cylinder and recoil shield, and then came down earlier to form one side of a "U" that exposed the trigger, and the depth of the U varies from really deep to so shallow that it's almost a gentle curve across the top of the holster;
3. A holster mouth with a fairly straight but sharply angled top that covered most of the cylinder but angled down to exposed the trigger guard;
4. A straight topped holster mouth that came across the top level or on slight angle that covered much of the trigger guard, while leaving some of the cylinder and recoil shield exposed;
5. A straight topped holster mouth that came across the top level or on slight angle but was cut lower to expose more of the trigger guard, and cylinder; and
6. Some variant of the military flap holster.
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Keeping the cylinder and recoil shield covered was more important in the percussion era than it was in the cartridge era, but it was I think also a matter of style as it persisted well into the cartridge era.
Tie downs were not unheard of but they were not very common either - certainly not nearly as common as they are in the movies. There is I think a very practical reason for this as it tends to be a PITA when you're on a horse as it holds the pistol at an unnatural angle when you're in the saddle. It's also not real comfortable when riding.
I've seen a couple of examples with a retaining strap, but they were also very uncommon and I lump them in as an extreme variant of the flap holster as it's seeking to accomplish one of the same goals as the flap holster.
I've also seen some old holsters with a hammer thong, but those have been holsters that date to the early to mid 1900s. I certainly saw more than a few of them growing up on a ranch in the late 1960s and early 1970s. Personally, I like the hammer thong, not so much for when you are wearing it, although it does keep it in place on horseback, but rather for when you've taken the gun off and are carrying it as it ensures the pistol won't fall out if it gets angled downhill. It's actually pretty practical and easier to mess with than a strap or flap.
