Yeah, I sure remembr it.  In fact, I think I still have one around here somewhere.  I also have or had an sfb one, but as someone pointed out, it isn't as sensitive
although it is a bit smaller.

Not sure about the Fowle one, but I also have one made by Bill Gary around 1979 or so which is really great.  It has a broader range than the Science for the
blind unit or the Conway one for that matter.

I also believe I may still have a unit made by Rick Joy sometime in the 1960s or 70s.  This was an interesting one as it was built intoa case and the chasis was
insulated as were the controls.  It had 3 separate comparators with a button for each so you could actually have 3 separate tones to compare against the one
driven by the meter itself.  It connected to the rig through a cable which could be disconnected from the unit itself.  The purpose of the insulation was to allow
the unit to be hooked across a meter which could be carrying high voltages such as the ones in the drake and at least some of the older Collins gear.

All these units really operated on the same principle as someone described in another message, but were different variations on that design.

Don  W6SMB