Hey all, Fred&Edna asked me to come over and see if I can provide any local insight, since I've lived in the Boston area for like 30 years now. I don't know whether I have anything relevant to say, though.
We moved here about 10 years after the murder. It was still talked about then -- still is now and then. The rumor has always been the one that's mentioned several times in this thread, about a professor she may have been having an affair with, or who may have been trying to pressure her into an affair; quite a few people seem to think there was a drug connection as well. The rumors have not always named the same person, however
I've heard at least three names over the years.
The blackout is not unusual for Boston-area police departments. You can't get the time of day out of Boston LE. It might mean they were protecting somebody but it might also mean that with so many suspects, they didn't want to fuel any more rumors than are already out there.
Another rumor that's always been around: that many of the unsolved murders of young women around New England were committed by a cop who was a serial killer. I've never heard Ms. Britton mentioned as one of the cases, though Ada's case is.
People around Boston used to smoke like chimneys. If there was a cigarette butt, it's likely the person just lit up from habit, without even thinking about it.
The security issue and unlocked doors -- even today, people around here are casual about locking up. In 1969, it wouldn't surprise me if fewer than a quarter of the doors on any given night were locked. I don't have any real numbers, though, just an impression.