It probably depended on your PDP-11 operating system. On RSTS/E, the extension determined which run-time system was used to run an executable. Off the top of my head, there was BAC for BASIC-PLUS, SAV for RT-11, TEC for TECO, and I can't remember the RSX one. The systems administrator could install other run-time systems and assign extensions to them.
However, at this time, extensions were closer to being actual meta-data since they were stored separately from the filename. Again, for RSTS/E there were six characters for the file name and three for the extension. I suspect that many other operating systems of a similar vintage did things similarly, except for Unix which just had 14 bytes. Even early Unix though had things like