I have a 3" 10-6 with a nice, but worn parkerized finish. Were these guns ever produced this way or is it more likely that someone had it done down the line?
Took a quick look at the model 10 section of SCSW, didn't see a parked 3" M10-6 listed but never say never with S&W!
Is yours a RB or SB? How about a photo?
It's not likely to have been Parkerized (phosphate finish) originally. Examples of post-WWII revolvers with phosphate finishes do appear every so often, and are generally assumed to have been done outside the factory. Parkerizing is a fairly simple and straightforward process, and is within the capability of almost anyone to perform. Parkerizing kits for guns are available for home use.
I've never seen a factory parkerized S&W revolver. I have had a couple of mine parkerized by a local 'smith, a 586 L-Comp and a 3" 29-10, because I like the look and durability of the finish. That M10 looks great!