There's no way to know for sure, but Premier, Sonor, and probably other EU makers all knocked off Leedy snare drums back in the 1930's, as they were considered the world standard at the time and American jazz and vaudeville drumming had great influence on Great Britain and Germany. Peruse some Premier catalog pages of their production from around 1930's and you'll see that many of the snares are almost exact copies of Leedy - all the way down to the lugs and angles of the bearing edges.
George Way actually was sent to Europe to market the Quick throw strainer, with success to Premier and other GB drum companies that were connected.
So they were`nt copies, they were actually made by Leedy to their specs. and some were the original strainer un-changed. I don`t remember where I read this but I know I found it very interesting.
When it comes to Sonor, it get`s even more interesting. Throughout my reaserch I have learned that not only did Bill and Mr. link live near each other in Germany but met frequently and traded alot of ideas and drums.
The Leedy strainer of the 20`s is strikingly familiar to the engarved snare strainer Sonor produced in the twenties. Leedy changed the way it`s mounted but Sonor remained the same two screws mount frame throughout it`s production because they had their own foundry and made everything, both before the move and after the move. They enventually let go most of the the foundry and plateing station as early as 1969 that I could find.
Like you say,...it`s not written in stone but I`ve concluded that both Ludwig and Sonor traded parts and ideas and possibly Leedy with just a handshake or an OK without any form of contract or written agreements because Sonor had no intention of a US market so it wouldn`t really matter much. Sonor didn`t market to the US until the early 70`s.
So I doubt there were infrigements going on,..I believe it was all done with permissions. This is why MIJ is criticized for doing it without permissions !!