You appear to have a Trooper MKIII with large target-style grips & a wide-spurred hammer.
It is not a Trooper .357.
The MKIII has a completely different internal action, no parts (including grips) are interchangeable.
I don't know that there ever was a designation of "Target" or "Service" applied to the MKIIIs.
What else do you need to know about your gun?