[Milsurplus] Unknown Crystal Filters

[Hubert Miller]

I found this filter adapter, built onto a board stock piece ontop a Vector 9-pin tube plug.
The filter has a typed label on it, "2.1 kHz", is  1/2 x 1/2 x 1-3/4 inches, has small input and 
output transformers.  The ground is pins 1,8,9  and the input and output are 7 and 2. Do you
know, was this adapter featured in any magazine article you saw, like a plug in adapter, or
for some HB receiver project ?

I am sifting some of my stock of parts right now. I also found a Fox-Tango filter for 3396 kHz
( I know, but the last digit DOES look like a '6' ) marked "Model 33H ??"

Also a "Sherold Filters Inc. 1757  Part No. 001-0360"
Also a "C-F Networks Model No. 21  5-35"   ( spaces in digits, as original )
About the latter 2 above, there is nothing on the net.
Any comments?
tnx,  Hue