I bought my first 12/15 at a gun show in upper New York state about 20 or so years ago. Think they may have been brought in by Navy Arms at that time. But remember seeing them for sale in the shotgun news prior to that. Think I paid $125 for it and had offers to buy it before I left the show. Frank

Al Freeland was importing and selling Martinis in the 50's- 60's as I recall.   But the ones I remember were the later Marks and Internationals.  I don't know when or if the earlier ones were imported commercially in any quantity or who might have done so.

Just for context remember that the Peabody-Martini variations were being used with good results in long range shooting as well as some midrange and schuetzens on both sides of the Atlantic and some of the german american smiths were building on Martini actions.

So as Competition-quality rimfire ammo came into being Martini actions were not as far off the radar as we might think at this remove.    I'm not sure how the English martinis fit into the story though.
  I'm thinking that the topic of the evolution of the 22 rimfire singleshot target rifle would make a very interesting article or two for the Journal since it would involve both the development of the ammo and the creation and popularization of the guns as well are the organization of competition.