Info on Keithley analog nanovoltmeter models 147 148 149

[gbaddeley]

Hi everyone,
I have pulled a Keithley 147 and a 149 out of my old test equipment storage.
Manuals are available on the 'net, but does anyone know when they were in production?
The 149 seems to be early 1960's. The 147 (and its 148 partner) seem to be late 1960's through to early 1980's.

Also, when did the 147 / 148 change from a 2 stage tube front end to a 2 stage FET front end?
My 147 has early 1976 date codes on components and is FET front end.
It looks like Keithley retained the main PCB with traces for the 2nd stage tube, and put the 2 stage FET front end on a little PCB in the shielded input compartment.

Cheers, Glenn.