The First Model M1904 McClellan saddle

Around 1905, the M1896 saddle changed in a significant way. This was the change from black leather to russet, which was an Army wide phenomenon for all leather equipments. The saddle and it's construction remained essentially the same. With the first model 1904 [hereafter referred to as M1904(I)] saddle,  there were changes in the stirrups and stirrup straps. Instead of the one thickness of heavy leather used to form the hood, held by brass rivets and burrs, the new stirrup hood was formed of two medium thicknesses of russet leather, held by black tubular rivets and caps.

The rigging safes were also doubled in the same manner. All metal hardware was also finished in brown jappan for enlisted men, and bronze for officer equipments. This only lasted for a few years until it was replaced by a more durable black chemical finish.

The stirrup straps continued to be constructed with reversed buckles and reinforced sections where the stirrup transom would rest  (same as M1896) . The reversed buckle allowed the stirrup strap to be twisted when assembled. Depending on the direction of the twist, the openings of the stirrups on both sides could be made to naturally face the rear, reducing the stress on the rider's legs. A reason for this can be seen in the increasingly heavy weight of strap leather that was being used for stirrup straps. The stiff 14-15 oz. leather used in the earlier models tended to turn the stirrup foot openings outward. The rider, if they dropped their foot from the stirrup, would have difficulty in returning it to a outward facing stirrup, especially in the flurry of mounted action.  

Many of the M1904(I)'s were later converted to the new M1904 specification still hunting down concrete documentation on this, but it may be the case that actual new M1904 tree construction was delayed several years (until 1908) as these earlier models were "recycled". Most of these recycles were unaltered Type I seats, with the later adjustable rigging added.

Here are some pics of a nice M1904(I) seat and pommel plate (lettering style is typical of M1904(I)'s)

Note the squared rigging ring safes and intertwined quarterstraps.