A forage gun is a shotgun on a military action once issued for hunting game in isolated outposts. I have seen them in Trapdoor Springfields and Remington Rolling Blocks, but don't know about a Martini.
The .402 Enfield was the last gasp of the black powder single shot infantry rifle, immediately obsoleted by the .303. So nearly all the .402s made in 1888 were converted to .577/.450 for issue to colonial troops without having to deal with another caliber.
I don't know for sure about the smooth bore business. A lot of British rifles were reamed smooth as improvised shotguns. Some supposedly as guard guns where any weapon would keep out the riff-raff without giving a traitor a long range rifle. Some to get them off a British Firearms Certificate and let them be listed as a shotgun for a British collector under their repressive laws.
I am not a M-H expert but a google on 1919 martini henry found the following:
"1919 Date - Here's the biggest red flag. It's almost unheard of to see a genuine Martini-Henry with a 20th century date of manufacture. A SCANT FEW were made up to 1901,