Jack's thingee is a passive CW/SSB AF filter made from telephone 88mH and 44mH cores and large silver mica caps. It's from an early 70's QST article and was featured in several ARRL Handbooks after the article appeared.

It was designed to plug into the AF speaker print of the Rx and in turn drive the outboard speaker of headphones. it had a very good bandpass space for each mode. It was actually two separate filters switched into the line for the intended mode of operation.