Joe, great gun at a bargain basement price. The markings in the yoke area are assembly or fitters marks. I agree that the grips are not original to the gun. You should also find the serial number on the rear of the cylinder, bottom of the barrel, inside the extractor star and on the back of the yoke, look through the cylinder holes to see it, a bright light will help. Assuming all the numbers match I would be inclined to order a history letter for the gun from S&W. I think the $50 for the letter would be a wise investment for a scarce gun such as yours. Here is an interesting thread you might enjoy about pre-war guns like yours.
2 inch, Pre-war M&P observation