I looked at my 199s and neither had the Keithley connectors in them (I have them somewhere). One had a 3 terminal connector in it that is the correct mating connector so I took pictures. The Keithley connectors are the same but 10 terminal, and they have a tongue to make it easy to pull out. With this little 3 terminal connector, I had to reach my fingers in the slot to get it out. Pitch seems to measure 5mm. Markings on this connector are:
PTR
>PA6.6<
AK 950
I got my Keithleys many years ago, and I have no idea where I got this other little connector. If I run across the Keithley connectors I'll post a picture, but the type I show here mates correctly with the meter.