[Milsurplus] RCA mechanical filter, was Boulevard Receiver

[Unserviceable but Repairable]

Whatever the case, Zenith not Collins invented the mechanical filter

Z published in Radio News, '46 throwing idea into public domain.

Collins lawyers worked on semantic twists to get a patent but to
patent office's credit, was never to be.

Certainly Collins got the lion's share of the business traffic on
subject filters

Can supply references