Looks like it was expensive when it was new.
Yep, just an example of how it could be done.4pyros
The circuit you show (post#15) will not give the tone discrimination the OP wanted for a range of 0.1 to 5 Ω. A 5 Ω change in the top resistor is only a 0.1% change in discharge time and roughly only one tenth of that in the charge time. The OP wanted the frequency to more than double over that range.
John