For a period holster design, I'd recommend the old AL Stohlman pamphlet on how to make holsters from Tandy.
http://www.tandyleatherfactory.com/home/department/Books-Patterns/6055-00.aspx?feature=Product_99
This contains a number of period type holster patterns and details on how to assemble. While the cover shows cowboy, most holsters are early 1900's designs, not western.
These are the same types of holsters that would have been used by law enforcement in the early 1900's except the very early versions wouldn't have had any type of safety strap. Snap straps weren't in use that early and didn't appear until sometime in the 1920's I "think".
The 1900's LE holster would have the finger cut-out around the trigger, but would enclose the rear of the trigger guard.
Color would usually be plain black with no carving or basket weave stamping, and not much in the way of molding to fit the gun, certainly not the close molding you see on modern holsters.
These old holsters were basically a one or two piece leather envelope, often of thinner leather than used later.
In all, the same general holsters were used from the 1890's to the later 1930's with about the only changes being the snap safety strap being added.
Construction would be very simple, usually with the belt loop simply folded over and stitched, and either a very thin welt on the holster body seam or no welt at all.
The patterns shown in the Tandy book would be totally appropriate.
