28602R was probably made in 1977, but may have been made in 1978.
The year 1977 started the "R" series at 01001R.
1978 ended the "R" series at 59999R.
As always, Colt serial number ranges are "complicated".
Your serial is in the Police Positive Special range, but Colt was intermixing all "D" frame revolvers in the same serial number ranges.
The finish isn't stainless, Colt made it's first stainless steel small frame revolver in 1995-96 as the Colt SF-VI.
Colt did make a few limited runs of "Coltguard" satin electroless nickel in the late 70's, and may have made a few in satin chrome before that.
If the finish is mirror shiny, it's probably standard Colt bright nickel.
