The PDP-10 architecture was an almost identical version of the earlier PDP-6 architecture, sharing the same 36-bit word length and slightly extending the instruction set (but with improved hardware implementation).
It was the machine that made time-sharing common it looms large in hacker folklore because of its adoption in the 1970s by many university computing facilities and research labs, the most notable of which were MIT's AI Lab and Project MAC, Stanford's SAIL, Computer Center Corporation (CCC), and Carnegie Mellon University. The PDP-10 was a mainframe computer manufactured by Digital Equipment Corporation (DEC) from the late 1960s on the name stands for 'Programmed Data Processor model 10'. File extension sit is mainly associated with Fortran-77 compiler (Load-and-Go Fortran-77 Compiler) developed for DEC PDP-10 mainframe computers developed by Steven's Institute of Technology.
