Murray,

For more on these instruments you can get *The History and Artistry of National Resonator Instruments* written by Bob Brozman.

The National Company (who also made resonator mandolins and ukuleles)included the Triolian (a wood-bodied single-cone resonator) line and the Duolian (steel-bodied) single resonator) line. Both of these lines included the f-holes in the design. The Duolian was a bottom-of-the-line model and therefore was quite popular in it's time ($32.50).

The three-cone or tri-cone guitars National built tend to have an open-grill work above and below the neck instead of the f-holes. These grills give them a very art-deco look. These were all steel-bodied guitars. They were targeted at professional musicians and were naturally quite more expensive.

Roger in Baltimore