Doing a little fun research, it looks like the patent on this was first granted in 1983 and assigned to Pepsico, who at the time was the owner of Wilson Sporting Goods. Patent #4,398,965 claimed an iron with a "front striking face and slot space rearwardly from the striking face" and that this would "store energy when striking....and transfer energy to the ball as the ball leaves the striking surface". This appears to be a follow on from their work patented in 1976 using a flexing slot on the insert in their wood clubs.
Since the original Pepsico patent on the iron slot was granted in mid-1983, they would have received 17 years of protection on that concept and their patent would have expired in summer 2000.
Subsequently, a number of other patents with various manufacturers have played off this theme as well. Nike has patents for a "bridge slot"; Callaway has some patent activity for the slot concepts, and Taylormade has some patents going back to 2001 on similar concepts.
The Wilson patent, however, wasn't the first in this field. A 1976 patent with Shamrock Golf Company had a similar concept with the slot extending almost through the club, and even before that there was a patent granted in 1962 with a slot going through the club.
