The book says 1966 is most likely.
I picked up one of these (later than yours, mine is ~1974 vintage) just this summer and I've had an awful lot of fun with it. I've been shooting .357 Magnum for a long time, but my 6-inch 28-2 handles my heaviest loads better (and more pleasantly) than any of my other .357 Magnum revolvers. I had been pining over a .357 chambered Ruger Redhawk for quite some time, but those were made in notoriously short supply. After the first range trip with my 28-2, I quit worrying about ever buying a .357 Redhawk.
I find it to be just a bit of an ugly duck, but it's a workhorse and sure is a lot of fun to shoot.