This "one size fits all" rifle was called ".303 caliber, Rifle, Short, Magazine, Lee-Enfield, Mark 1", or, in short SMLE Mk.I, where "short" referred to the length of the rifle.
In 1903, they introduced a new design, which improved over the older Lee-Metfords and Lee-Enfields in some important respects.
The main improvements was the introduction of the "universal" rifle idea.
The front sights were protected by the two "ears" on the stock nose-cap.
Latter the front sight were changed to post type, and the rear - to the U-notch type, and since the introduction of the No.4 rifle the barrel-mounted open rear sight was replaced with peep-hole one, mounted on the receiver, which made the sighting line much longer and improved the long-range accuracy.
With the outbreak of the WWI, British troops were still armed with the "poor" SMLE Mk.
III rifles, which soon turned far from any "poor", giving some hard time to the Germans. III was a really good rifle, quite accurate, reliable and suitable for rapid and accurate firing.
The common thinking of the period was to issue the long rifle for infantry and the carbine for cavalry, artillery and other such troops.
The Brits decided to replace this variety of sizes with one, "intermediate" size, that will fit all niches.
Australian, Canadian and Indian factories turned out more than million of the No.1 rifles with various improvements, which were used during both World wars and thereafter.