I'm not sure of the exact sequence, but those models pre-date the GP100 series. The Security Six had adjustable sights and directly precedes the GP series. The Speed Six had channel sights. I'm not sure where the Service Six fits in there as I don't know anything about it. In fact, I've never seen one.
I had a stainless 4" Security Six and it's the revolver that I miss the most. They are stout, tough revolvers. They are generally available for decent prices. Just get one in good shape and you'll be very happy with it.
“If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen.” ― Samuel Adams