Alpha Dual Potentiometers - Linear Taper, 8mm Bushing
24 mm carbon composition
1/4" diameter solid shaft
500 VDC rating
I found them here:http://www.tubesandmore.com/
They have them in 250 and 500k. Dual, linear, single shaft, the only drawback is the bushing is 8 mm as opposed to the usual 3/8", but as long as you're starting from scratch you can drill the hole you want. Hopefully they are easy to open up and swap the resistor wafer.
"Do not write so that you can be understood, write so that you cannot be misunderstood." -Epictetus