Due to recent questions about them, I looked at few No 15 sights to document some of the variations. Based on information from a discussion a few years ago these seem to have came into use about mid 1913. Catalog No 50 circa 1912 has only the No 14RM rear sight listed for 1899's. Catalog No 55 circa 1914 has the No 15WG as 'furnished on all Model 1899 rifles' and the No 14RM as 'adaptable to Model 1899 rifles'. In catalog No 60 from around ~1919 to 1920~ the No 20SR became the standard sight for 1899's and the No 15 WG was listed as an option - this is the last retail catalog to have the No 15 WG listed. It's listed in parts catalogs through Sept 1929 being dropped from the 1931 issue.
The early ones are stamped patent applied for and the dovetail is constructed by curling the sheetmetal of the main part of the sight over a piece of thicker metal, the later ones have a patent date and are made by folding the sheetmetal based on a method in a patent held by Stevens. I do not know which change came first, I've not seen one of early construction with a patent date, which I would expect, or a later one without. The same construction changes are seen on the No 21B (correction 25B) sight that was also used though 1920.
These sight have such small notches they do not look very usable, they also sit quite high requiring tall front sights that would be more prone to damage. The two modified ones have notches about as big as they can be made and still have the windage work.
The bottom one is an early 2 piece. The pictures do not show it well, but the bottom two are bright high polish blue and the top a duller matte like finish.
The patent for the No 15 (Anyone have a tang sight version?) and the Stevens patent -
The Stevens patent for folded construction would have been acquired by Savage with the purchase of Stevens in mid 1920, but it appears they may have started using it earlier. All of the No 20SR sights I've seen are of the Stevens patent construction, so unless the earliest ones were different construction, or Catalog No 61 is later than the mid 1920's, Savage did this before acquiring the patent.
The elevators are formed with a rounded bottom and then the inside is milled flat, some more than others, so the thickness varies, some are very thin in the middle. The ones with the small tabs are the later version.
These sights show up quite often, more than 6 in the last couple months on eBay. but they also show up quite often with modifications that you should watch for. The parts from the one on the left were used to repair another sight and the two on the left will make one good one.
This shows how thin the parts are in spots, often there is an attempt to make the sighting notch larger and deeper - this usually results in cutting into the fixed inner part. The thinnest part on top, over the windage screw center, is less than 0.025" thick (where the hole is in the one shown above), the bottom is only 0.31" thick at the sides and the flat between is very thin.
The most common modification is to the V, here's a recent example of this that was on eBay, it also had modifications to flatten the bottom... which didn't work out well.
Details are of the above sight, the V is filed deep into the fixed inner part, so no more windage adjustment, and the bottom was filed flat causing half of it to fall off allowing the rest to tilt -
All examples of these I've seen have rounded bottoms and I've wondered if they made a flat bottom version for octagon barrels or if the round bottom ones would work. Looking at a round bottom on an octagon barrel it does not look to work very well, even thought these are listed as 'furnished on all 1899's' I think they still would have had to use an earlier type sight on octagon barrels. The round bottom sits on the edges of the flat making them sit higher than on a round barrel and trying to flatten out the bottom would weaken the sight because the metal there is quite thin.