Fudgie Ghost, The grip that will fit your #150 Security Six is the Pachmayr Large Presentation Grip for the Ruger Security Six.
This is a Target Grip, the Model Number is RS6-L 03177.
This one will fit your Ruger better than the Combat Gripper Grips from Pachmayr. Also, they fit better than Hogue's. I should know, I bought 1 of each grip available for the Security Six and tried them all until I found the one that fit the best.
However, you will have to modify either the grips or those blasted roll pins near the bottom of the grip frame. I grounded the pins down on each side until the grip fit.
I now remember back in the Seventies, I had to do the same thing. I also used the same grips back then.
The best & cheapest source for grips is cdnninvestments.com.
Grips in my local stores cost $20 to $30 a piece but at CDNN Investments they are only $6.99 each.
Best of luck with your Ruger, they are great, I carried one for six years in Inner City Detroit and it saved my life more than once.
Now, I need to find enough Ruger Security Sixes to fill all these grips.