I'm looking at a Ruger Security Six Low back in .357 magnum. It's a 4" blue with the original grips. It's got a little wear around the muzzle in fact it has a couple of pretty good dings on the under side of the muzzle. Otherwise it looks pretty clean. I believe it was from 1973. Now this is where it gets interesting, at least to me. It is a Security Six but is a fixed sight like a Service Six. I'm told it is a transitional model. What is the difference, if any , in value for an older model and a low back as opposed to a later model. Then what would it being a fixed sight Security Six add to the value. Are these pretty rare? Looking for an approximate value.