My guess is that it is post patent. As you mentioned they were made for many years and keeping all the patent no's. on tools required a lot of die maintenance. For this reason when the patent ran out, then the patent no's. were left off. Made the forging dies cheaper to maintain.
As an aside, I got a one-hander at an auction this year, similar to yours, that was made in Denmark.
There are a lot of different one-handers out there...even Sears had/has a knock-off of the Masterwrench.
Good luck with the collection.
Mike