Hi,

May 1964 QST had a product detector for the HRO receiver NBFM socket.
(I presume that is similar to to the NC-183D NBFM socket).
The article was by Loren Windom (of "Windom antenna" fame).

73,
Ed Knobloch

Sometime in the 1960's or 70's I bought a similar kit at the old
Portland Radio store in Portland Oregon. I installed it in a National
HRO-60. Worked good, as I remember. I don't have a clue as to who put
the kit together. It was a commercial enterprise of some sort. It was
one of several kits in bags on a rack.

I would vote to just leave it in the radio along with any information
you may discover, or a schematic you might draw out.

My $0.02 worth!

Paul, KD7HB