Security six question?

[1 LT MPC]

Anyone know when Ruger changed the grip frame on the Six series revolvers? I have a Security Six with a 153 serial number prefix ( maybe around 1976-77) and I'm wondering if it's an older style frame.

 

[smee781]

I think 151 and up is the new style

 

[GrandmasterB]

150 is old style, 151 and up is new style.