Welcome to the forum.
I own S159084 that is a pre model 27 and S159461 that is a 1955 .45 Target. The pre 27 shipped in September of 1956 and the the 1955 .45 Target shipped in February of 1958.
Assuming that your gun is not model marked in the yoke or barrel area then I would guesstimate that yours is a late 1958 gun.
Chances are that as others have stated, your gun is either a pre 27 or a Highway Patrolman. If a pre 27 it will most likely have a bright blue finish and if a Highway Patrolman it will have a dull finish. The barrel of the Highway Patrolman will also be marked as such so it should be very easy to tell the difference between them. The Highway Patrolman went on to become the model 28 and is basically an economy version of the 27.
Since you didn't remark about the barrel being marked Highway Patrolman, my money is on this gun being a pre 27.
Hope that helps.