As above, Colt did offer satin Electroless Nickel finishes on the Trooper Mark III.
At various times Colt called this Electroless Nickel, and "Coltguard".
Satin EN Mark III's are not too common, but not at all rare.
No Mark III guns were ever produced with stainless steel hammers or triggers. The hammer and trigger on the poster's gun will be plated.
As far as I know, there is no price premium on the EN finish itself.
Some versions with the EN finish are rare.
As example, the Mark III in .22 Magnum, 8" barrel, and EN finish is rare, and this is due simply to the lack of demand during production holding numbers produced down.
Bottom line is, a common barrel length and caliber Trooper Mark III with the EN finish, is neither rare, nor worth more than a blued version.