Any idea of the date that might have been installed?

The house was built in the 70s

Put a meter on the wires and try throwing the switches in various combinations.

Possible that if they put 3 way switch wiring in, they just used a length of 14-3 to feed the light with one conductor unused.

In the 1950s and earlier, it was not unusual to install two runs of 2 conductor NM cable in lieu of one run of 3 conductor NM for 3-way circuits. One of the conductors would be left unused. Some electricians continued the practice out of habit even after 3 conductor NM became more available and the code required all conductors to be in one cable.
Another possibility is that the installer simply did not know what he /she was doing and just gave up on it.

Another possibility is that the installer simply did not know what he /she was doing and just gave up on it.

I have been away from this forum (and on to social networking) for way too long -- I just found myself looking for the 'Like!' button.

I fear that experience has turned me into a misanthrope. The comment about the installer came from an experience on a volunteer rush house building project. 8 men were going to do the electrical in an afternoon and as much of the night as necessary. At least 3 were licensed contractors and two of the others were people who thought they knew much more than they did-- if you know what I mean. Two circuits connected together in wall box in bedroom-- cost me a ruined pliers plus nasty shock twice while trying to find the problem. Wall boxes all over the house with blank covers because the circuits simply did not work. I told the lead man that we needed to test everything before the drywall went up-- but he did not agree. I always test my own work while the walls are open. Some do not like the delays that involves.