Nighteyes, he's not saying the top one is 16", he's saying the top one is 10" and the one further down with the canvas scabbard is 16".
Guess I'm not being clear. I am not saying the top one measures 16 inches NOW. I'm saying that, if the top bayonet's current blade length is 10 inches,
it was cut down from an original blade length of 16 inches. Here is how I know this.
The first is the "SA" mark. That stands for its manufacturer, Springfield Armory. Springfield Armory only made 16-inch, Model 1905 bayonets. Any 10-inch bayonet with the "SA" marking was cut down to 10 inches from an original 16 inches.
The second is the date on the blade, which appears to be 1918. That again marks it as a World War I Model 1905 bayonet, which all had 16-inch blades. Even if the date was 1942 or 1943, which would make it a Model 1942 bayonet, all of them also had 16-inch blades. As I said before, many of these 16-inch bayonets were cut down to 10 inches.
Third, and last: only five companies made the 10-inch bayonet known as the M1 Bayonet. They were UFH (Union Fork & Hoe), AFH (American Fork & Hoe), PAL (Pal Cutlery), UC (Utica Cutlery) and OL (Oneida Limited). Of all of those, only the first few made by Oneida had 1943-dated blades -- they used up their dated but as-yet unmilled blade blanks for the Model 1942 bayonet before moving to the undated version. All of the other manufacturers used undated blade blanks. This is why I said the UFH-marked bayonet was probably manufactured with a 10-inch blade rather than being cut down from 16 inches.
There are many things I don't know, but I do know US military edged weapons - both issue and private-purchase - from just before WWII through Vietnam. I've been collecting and dealing in them for several decades now.
But hey, don't take my word for it. Consult the works of any number of experts including Mike Silvey, Gary Boyd, Frank Trzaska, and the late M.H. Cole.