Gene,
The gun was originally London proofed, but has been Birmingham nitro proofed 1904 to 1921 (no date stamped viewers mark introduced 1921).
Depending on how it has been treated since in last 102 years it may or may not be fit for use with 2 1/2” nitro cartridges.
At least singles tend to have thicker barrel walls than doubles.
Webley and Parker-Hale both made similar.410 adapters, but not as long as this one which skeettx correctly identifies as a Savage Four Tenner.
Unless the 20 bore tube is very grossly pitted you would be unlikely to have any problems with putting any factory .410 cartridges through one of these adapters.
That is what I plan to do with my 12 bore Army and Navy hammer gun when I get round to putting it back together.
