I cheated. I took the screw to the local machine shop and had them alter a S-K big-fat-stubby screwdriver to just fit including the flat grind on the blade.
Some years later I had the custom knife maker make me a chisel grind huntng knife with, you guessed it, a scewdriver tip that fits the Ruger scope ring slotted attachment screw. Before you get all weird, it's a copy of a Navy screw driver/knife isued in WWII. It also works great for turning in Dazu fastners.
I'm afraid to look at the special tool from Brownells because I know in my heart of hearts I spent abot 4 times what I needed to.
On the other hand I got a one off tool. made to my exact specs, pride of ownership and I really do own a bunch of Rugers, which is how I justified it. It isn't like I'll ever need another one.
In early years I was known to use a quarter held in a pair of vise grips!!!
Ross