The DECSystem20 (and the PDP-10 before) were 36 bit two's complement machines. You had to go back to the PDP-1 if you want one's complement in the DEC line. The CDCs and UNIVACs were the only ones that were still kicking around in my era.

Just to confirm, I pulled out my PDP-10 Hardware Reference Manual; Vol I - CPU
(EK-10/20-HR-001), and it does indeed say (pg. 1-12): "The fixed-point
arithmetic instructions use 2's complement representations to do binary
arithmetic." Selah.

Back in school, we had our machine organization course. When we learned about 1's complement, the professor said "I've used a lot of machines that had this. You will likely never see one with it. There are no operational machines on campus with that."It stuck with me. We had a TOPS-20 machine... the odd turn of phrase was due to a professor that had a board of unknown origin hanging on the wall that was a rumored to be a CDC or similar... ah, the mid 80s...

Warner