Cliff;I will go on the high side of Bob's estimate,or $400, W/O seeing your gun in person,it has 2 things in its favor; First,nickle finish, In over 20 years of collecting mostly E frames(like the O.Police),SAAs and the big New Service D.A. Revolvers,I have seen "2" nickled O. Polices,out of 100's! One was a NIB specimen a dealer was asking,and got,$500,for at a show over 10 years ago. The other, was one that 3 young people brought into a gun shop,I frequented,around 12 years ago. They wanted $175 for gun,and dealer offered them $125-no sale. I came very close to following them out to parking lot and pulling out the $175,from my "stash" of bills
,but figured I might get attacked when they saw the roll of billss,or that gun was "hot", Probably should've bought it,and it had great real stag stocks.
Over on the S&W Froum,we've discussed that 4" S&W M&Ps are far scarcer on the pre war guns than on the post wars,and I think this is true for Colt's competition to the M&P,the O.Police. 6" models were very common before the war,but after,the 4" took over,and the 6" models were far less popular. A variety of reasons for this,too long to add to this already long post.
My guess on the markings is probably a P.D.,but it also could be from a gun issued to a plant guard.
Only problem with nickle guns,is that any "nickle loss" shows up more,and distracts from the guns overall appearance,than a blued gun,wich seems to show less obvious wear,especially if the worn area has taken on a nice "patina".
Good Luck,and BTW,glad you have the original stocks,and I bet if you added a grip adapter,especially a polished stainless steel one by Tyler,you'd find the original stocks would feel better with your hand fit.