Your picture isn't showing but I assume you meant right side decock lever.
S&W used (for the most part) four different methods across all the 1-2-3rd Gen pistols. Your 645 is a 2nd Gen pistol. In order, they did:
a) single side decock only
b) right lever affixed with ugly Philips screw
c) right lever affixed with better hex-head screw
d) right lever affixed spring loaded pin retainer
Option "d" was obviously the best.
For you to go with option "d" on a 645 would mean that you need to source the entire drum-shaped unit from a 3rd Gen pistol. I know that Brownell's sometimes has these parts, I think you are looking at some $60 or more to get them -if- you can find them. You need the entire left side/right side assembly, you cannot simply attach a 3rd Gen ("d") style right side paddle to a 2nd Gen 645 pistol, sorry.