The 4/669 side panels fit the frame openings without modification

The back strap, being designed for a double stack frame, was obviously too wide.
I removed an equal amount of material from both sides until the back strap was the same width as the 3914 frame.
I then fabricated a wider "U" shaped piece of springy steel to fit inside the back strap (over the plastic stud) to retain the side panels in place of the original, narrower "U" shaped metal retainer.
Lastly, I fashioned a plastic spacer to fill in the area at the top of the back strap which positioned it tighter to the frame.

For those with sharp eyes, look closely at the angle of the grip panels as compared to the angle of the grip frame.

As you can see and I mentioned in an earlier thread, (3rd Gen grips on 2nd Gen Pistols (with pictures)) S&W straightened the grip frame approximately 2 degrees more vertical on the 3rd gens as compared to the 2nd gens, yet you never see or hear that mentioned when the upgrades to the 3rd gens are listed.

John