Anyone ever heard of a National NC-57 look-alike kit (K57B) for the US Navy?

[Mike Langner]

Hello everyone !

I'm currently checking the tubes on an NC-57 look-alike.  But the look of
the cabinet is where the similarity stops.  This receiver is in a cabinet
marked Navy nomenclature CY 677/U furnished under contract NO bsr 42174. It
is serial number 939.

The tuning capacitor and bandspread capacitor each are 2 section with both
sections of the two capacitors having identical capacity, although needless
to say, the bandspread capacitor has much less capacity than does the main
tuning capacitor.  The two sections of the capacitors being identical would
appear to make oscillator tracking impossible, which makes me wonder of this
thing is a TRF. But if it's a TRF, why would it have a 12SA7?

Tube complement is 12SA7, 12SG7, 12SL7, and a metal 1632 (6F6 like, it
appears) audio output tube.

Front panel controls are: Main Tuning, Bandspread, AF gain, Pitch,
CW/PHONE/ANL, Bandswitch , Send/Receive. There is no RF gain control.

Frequency range is .50 to 36 mHz.

Rear panel contains the familiar screw-type balanced/unbalanced antenna
connector and an SO-239, a round two-pin plug-and-socket that I suspect is
for connecting an external speaker, and a male (not female) octal
chassis-mounted socket (plug!).

Next I'll take the bottom off the unit and peek inside.  The unit does have
a standard brown 2-conductor power cord with a standard American (Edison)
plug on the cord.

There are two metal cubes atop the chassis that look like IF transformers.
Two transformers are mounted atop the chassis as well as one transformer
mounted on the speaker -- audio output tube plate to speaker transformer.

Several tube sockets are empty.  They have paper stickers pasted over them
with the words "no tube used here."

A plate affixed to the top of the chassis by National contains the National
logo, the words "Receiver Construction Kit", and in smaller print, the words
"K57B."

Anybody know anything about this interesting unit?

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