From the interview of Richard M. Stallman, I learned that when Richard Stallman joined the Artificial Intelligence Lab at Massachusetts Institute of Technology in 1971, the community of programmers (or say hackers) at AI Lab used to run a free operating system probably developed by themselves.
Now, from the history of Multics I learned that in early days of computers, (in 1960's) Time Sharing system, more precisely CTSS (Compatible Timesharing System) developed and later on (around 1965), Multics had been developed by joined effort of MIT, Bell Labs and GE. Group of developers moved from CTSS to Mutics at the end of 1968. Bell Labs withdrawn the development of Multics in April 1969 because of frustration but Ken Thompson (who earlier contributed in the development of Multics) started developing another system which called "Unix" in 1970.
So, I want to know which is the operating system and more importantly which is the kernel that hackers at MIT AI Lab used to run in 1971? Was it CTSS or Multics or newly developed Unix or something else developed by themselves?
P.S (History goes...) Later on, that operating system at AI Lab started to become non-free and free community died around 1982 which inspired stallman to start a new community for free software, GNU in 1983.