I have a 1914 Midland catalog. It offers .410s available in the new 2 1/2" cartridge at no extra charge if requested. So, I'd say 1913 is a good estimate.
The guns are listed in the For Ladies and Youth section of the catalog.
I have a no name British .410 double with sidelever and central hammers. It weighs in at 4lbs 2oz and sports a whopping 15 7/8" straight stock and 28" steel bbls.
There are Birmingham proofs with the 410 C in the triangle. It is not nitro-proofed. The SN is 76. The UK guys tend to believe that lacking the nitro-proof is typically pre-1896 when nitro became optional. It is definitely pre-1905 when nitro became mandatory. (If memory serves.)

Joe