Two Different Grip Straps...
The old, or original style, which had a definate 'bend' in the back strap, and the newer style, with a smoother curve.
I don't have any pictures.
This question might go better in the Collecter's column. I bet Harley has more precise information.
***Go to GunBroker.com; search revolvers for 'Security'. There are photos of both early and later style grip frames.
Slick 6; thanks for filling in what I didn't remember. I was issued a Security Six in the Border Patrol some twenty-seven years ago... I should really get one.