I actually did not hear Smellie say that a 1915 rifle is rare BECAUSE there were only two factories. (And yes, there were only 2 proper English factories at the time). I heard him say that 1915's are rare because they were really only beginning to ramp up production. If you look at the numbers below you can see how big the jump was in 1916. A big jump from 1914-1915 as well. But the numbers clearly show how long it took to really ramp up wartime production. But there is alos another reason why pre 1915 rifles are more rare than other wartime production SMLEs. Namely that the 1915 production would have worked it's way through pretty much every major action of the war, and would be more likely to have been lost, damaged, or scrapped.
Royal Small Arms Factory, Enfield
1903 to 1907 Mk 1 and Mk 1* 193,644
1907 to 1912 Mk 111 100,000
1913 Mk 111 30,000
1914 Mk 111 51,576
1915 Mk 111 271,856
1916 Mk 111* 418,283
1917 MK 111* 640,113
1918 Mk 111* 623,330
Total: 1,681,726
Royal Small Arms Factory, Sparkbrook
1903 to 1906 Mk 1 14,640
Birmingham Small Arms Company (BSA)
1903 to 1906 MK 1 and Mk 1* 150,000
1907 to 1909 MK 111 50,000
1910 to 1913 Mk 111 30,000
1914 Mk 111 51,419
1915 Mk 111 and Mk 111* 275,927
1916 Mk 111* 435,212
1917 Mk 111* 468,447
1918 Mk 111* 345,732
Total: 1,811,734
London Small Arms Company (LSA)
1903 to 1906 Mk 1 and Mk 1* 65,000
1907 to 1918 Mk111 and Mk 111* 430,000
Total: 495,000
Standard Small Arms and National Rifle Factory
1916 to 1918 Mk 111* 275,000
Ishapore Rifle Company (India)
1906 to 1909 Mk 1* 3,000
1910 to 1914 Mk 111 30,878
1914 to 1918 Mk 111 and Mk 111* 136,800
Total: 170,678
Lithgow Small Arms Factory (Australia)
1913 to 1918 Mk 111 and Mk 111* 112,454
Grand total: 4,561,232