John Pedersen, a long time employee of Remington Arms, was aware that the US would be entering the war at some point. Concerned about the inability for troops to effectively fire on the run while attempting to cross "No Man's Land", he decided to start studying the problem of semi-automatic fire that would allow them to fire from the hip without stopping. However, he also realized that there would be no way the Army would accept a totally new rifle design, as they were already struggling to produce enough Springfields, contracting to produce millions of M1917 "American Enfield" rifle with Remington and Winchester and were importing Ross rifles from Canada for training purposes.
This led him to the final design, which replaced the bolt of the standard Springfield with a device consisting of a complete firing mechanism and a small "barrel" for the small round. In effect, the "device" was essentially a complete blow-back pistol minus a receiver/grip using the short "barrel" of the device to fit into the longer chamber of the M1903 Springfield. The mechanism was fed by a long 40-round magazine sticking out of the rifle to the top right, and could be reloaded by inserting a new magazine. New sights were provided at the rear of the device. The system did require one modification to the rifle however, a hole had to be cut in the side of the bolt area to allow the ejection of spent rounds.
By 1917 his solution was perfected, and he traveled to Washington, DC to demonstrate it. After firing several rounds from what appeared to be an unmodified Springfield, he removed the standard bolt, inserted the device, and fired several magazines at a very high rate of fire. The evaluation team was astounded, and an immediate secret classification was applied. To deceive the enemy, the Ordnance Department decided to call it The US Automatic Pistol, Caliber .30, Model of 1918. Plans were put into place to start production of modified Springfields, which became the US Rifle, Cal. .30, Model of M1903, Mark I. Promises were made to have 500,000 ready for the 1919 Spring Offensive. The use of the Pedersen Device in the 1919 Spring offensive was to be in conjunction with the full combat introduction of the M1918 Browning Automatic Rifle (BAR).
The Pedersen Device was also modified to fit the US Rifle, Model of 1917 (the American Enfield), and the US Rifle, Model of 1916 ( The Remington Mosin Nagant). Neither of those were ever put into production, although samples of both were made.
Nice video of your '03 Mk1. Your barrel was made by a gun company named High Standard. I think they were mainly a handgun manufacturer. They were to originally get a contract to produce the 03-A3 rifle during WW2, but it went to Smith-Corona (typewriter company) but all of the barrels for the S-C rifles were made by High Standard, even though they were marked S-C on the barrel. They made a lot of replacement barrels for the gov't, and they were marked H-S, with date, etc.