I assume you got the prints that are in the AK forums here... Otherwise, my Jigs are all over the country at the moment... Perhaps someone else can get you a measurement?
I followed the prints that I got here and everything looks good. I made the lower unit where the opening is adjustable in case I decided to bend some of the 0.050" flats. I haven't been able to find a dimension for the opening, though.
Anyone that knows, please help! Thanks in advance.