Crystal and ceramic filters are also mechinical filters, since they depend
upon the mechanical resonance of the quartz or ceramic.

I recall that some of the radio companies have advertised that their radios
had mechanical filters, when in fact they had ceramic filters.

As mentioned in the latest Electric Radio, Zenith apparently developed the
mechanical filter but decided it was not that useful and abandoned the idea
a number of years before Collins revisited it.

Colin K7FM