Well, I had some time today to re-visit this thread to see if I could figure out where I got those dates above...
In short, I can't!
Those are the dates I have in my notes for some tools in my own collection, but I can't seem to figure out where I got them. I speculate that it may have been the best estimate I had at the time for the tools in my collection, but I'm not even sure of that.
I think the best I can do is the revised dates of 1959 - 1993.
The name of the company changed from the earlier Diamond Calk Horseshoe Company by 1959 (AA says as early as late 1957) to Diamond Tool & Horseshoe.
AA says the company was sold in 1981, but I had thought it was 1983. It was, in any event, to Triangle Corp. (I don't believe Channellock was ever involved, John) who were in turn acquired by Cooper in 1993.
I'm not sure how the tools were marked when owned by Triangle, but I am almost certain they were simply "Diamond" when controlled by Cooper.