After some searching I now have a copy of the details of the patent that Longhi filed for his new construction of tubular rims (patent no. 416,249 granted 19 January 1934). I understand that the patent would have been in force for 25 years. Certainly Fiamme had 'BREVETTO LONGHI' stamped into their rims (probably until 1959). I have never seen a Mavic rim with similar stamping but do know that the diamond decals used on earlier Mavic rims carried the words 'LICENSE LONGHI'. Greg Softley of Cyclomondo has an excellent selection of these early Mavic rim decals available.