Here are some of my notes. Corrections or additions welcome.
MIT’s Project MAC had already leased floors 5, 8, and 9 of the building for professor, staff, and graduate student offices, and for computer rooms.
Here is how I recall the building in the min-1970s.
The first floor contained a restaurant, was it the Tech Square House? Also there was the Office of Michael Dertouzos, Chairman of MIT’s Laboratory for Computer Science (LCS) and the LCS Reference Library.
The second floor housed the CSC Computer Room; I was on that floor, as was Fritz Giesin.
The third floor housed clandestine government offices, the CIA and perhaps others.
The fourth floor was the CSC, laid out similar to the MIT Offices above.
Other floors had varying tenants at different times. The GE Multics team, the Cambridge Information Systems Laboratory (CISL), had offices on half of the seventh floor. IBM had some other offices in the building. The programming group that developed VM/370 was originally housed there. IBM Fellow Nate Rochester and Jean Sammet were somewhere in the building.Tech Square was reconstructed in 2001; 545 is now known as 200 Tech Square.