[Boatanchors] NC-300/303 converter cabinet questions

[WØQFC]

I have the converter cabinet that has the C6, C2 and C1 converters installed
but the actual unit does not match the schematic ~

The difference is on the panel that has the selector switch and terminal
strip for connecting the three converters. The schematic shows one ceramic
standoff where the coax that connects to the converter input jack on the NC
receiver.

The schematic shows one standoff but the one I have has two standoffs, a
coil with 7 turns, 1" long, 1/2" dia and #18 enamel wire and by-passed with
a 82 mmf silver mica on each side. The extra standoff, coil and caps looks
to be original parts.

The questions:

    Why the coil and caps?
    Was there different converter cabinets for the 300 and 303?
    The pictures from the National sales sheet
(http://www.virhistory.com/ham/nc300/303-sales-bw.jpg)
    shows covers on the bottom of the converts. Are these covers necessary
for proper operation?

Any help will be greatly appreciated!

TIA es 73


Gene, WØQFC

A.F.&A.M. #91 Lafayette, Colorado