I can't tell you what year, but might be able to narrow it down. I have an old M68 I bought for my step dad that I inherited. It's got a blade rear sight with a screw on either side and appears to be adjustable for windage, though I haven't had to do that. I bought that gun about 1981.
In around 1989-1990 I bought a M88 (stainless version of the 68). By that time, the rear sight was machined into the frame.
So, if you have a blade rear sight with a screw either side of it, early 80s. By 1990 they were a fixed machined rear sight.
That's the best I can do. That little 3" M68 is really accurate, nice little gun for IWB. I carry it some, but it's mostly valuable to me for sentimental reasons. It's a well timed, tight little gun with a crisp SA trigger and a so-so DA.