For a gun with a very limited production, there sure are a lot of them here on the forum. My 2 both have service stocks. But just to add confusion to the issue, one of them has a grip adapter as commonly seen on RMs. It came that way when I bought it from David Carroll a while back. I don't remember when, but if its important you can ask him.
Now for the OP here. The next thing you should do is letter that gun. It will tell you what kind of correct grips you need to invest in. If you can attend a large gun show, take the gun with you. Its best to buy wood by seeing how it fits the grip frame of your gun. During the prewar the factory would screw the wood to the gun, then finish sanding the two together. Its why the early grips fit so well. Then they'd stamp the serial on the inside of the right grip, screw them together and send them off for finishing. And the gun would go to final polish. All is not lost because sometimes you can get a nice fit just by luck.
I had even better luck with the gun supplied by David Carroll. Months after I received it I was in a gun shop that was a receiving FFL for a different gun. The proprietor was bored and he and I were the only people at the shop. He was running around looking for something else to sell me. So from across the room he found a red box. Back in that time frame I was collecting every Outdoorsman box I could locate. And when he held it up even from 40 feet away I could get an idea of the condition. So I yelled over that I'd take it. And back then good red picture boxes were worth about $100. He was joking with me about not knowing how much to charge, and I didn't care. So we were finishing up the paper work and he grabbed a bag to put box and gun in. As he was sliding the box into the bag, I noticed it said "Masterpiece" instead of "Outdoorsman". I was shocked. Suddenly I had a new Katrina trailer for my lonesome gun.
And a poster here was kind during that time frame. He'd found another Masterpiece box in an old barn he tore down. It was semi melted by moisture. He couldn't sell it so he sent it to me. But I was bad and passed it on to David, who had a place for it in mind.
An additional interest item is you large size nickel plated screwdriver. One of my prewar Masterpieces came with that screwdriver. But one of my Postwar guns, K166 also came in a box with the same screwdriver. Leads us to believe they just used the same style after the war until they ran out and began shipping the black oxide coated ones.
Now on to value. Above the values were guessed at as north of maybe $2000. I guess the value is at least $5000, maybe more. But only with a correct set of grips. Its worth whatever you've got to spend to make it correct. That value isn't going to hold with postwar wood, or with the "Plainclothes" or "modified magnas" you've pictured it with. A little expense now could double the value.