3/16 is the size for standard ROA,I have an original Ruger wrench but choose to use a 3/16th socket,they don't fit in the nipple pocket without taking down the outside diameter,can be done on a bench grinder but a 3/16th nut driver from online should be ideal without work if it fits down the pocket.
The Ruger tool isn't a great fit in itself anyway,a little large for the pocket and somewhat crude tool,the screwdriver on the other end is a sure way to scratch up the loading lever screw...imo.