Thanks to everyone for the wealth of information on this thread, it has been invaluable on my road towards fixing my very first Keithley.
I bought 2 units off eBay which were listed as working. The units did not work. Both were showing a slew of errors 304.2-304.7 Ohms current sources, 307.2 cal divider ad mux/10, 308.1 4 digit mode ad signal path. I'll refer to them as unit 1 and unit 2 from now on.
Unit 1 worked and was displaying somewhat plausible readings straight out of the box. The self-tests above failed and I tracked it back to a faulty LT1007 in the AD buffer. I replaced the LT1007 and the meter works!
Unit 2 powered on but was displaying nonsense. With inputs shorted the DCV reading would slowly drift up from 4V all the way up to 12V. The ohms ranges were also displaying nonsense, half a gig ohm for a 500K resistor. I have been pulling my hair out all week, I replaced U319, U325, U332, and U322 before I decided to swap the AD board from Unit 1 into Unit 2. Unit 2 passed with flying colours with Unit 1s ADC. This leads me to believe the problem resides on the ADC board instead of the buffer. I haven't scoped data out but from the strange readings, there's definitely data going back to the CPU. I'm at a loss as to how to test the AD board. Any help will be greatly appreciated. Thanks!
I managed to source DG211 and HEF4052BP off eBay for a reasonable price. As far as I can tell they are legitimate components. The item numbers are 324013465152 (HEF4052) and 292514792305 (DG211)