UPDATE 2/11/2012: After thinking carefully about all the points raised in the very thoughtful comments on my initial post, I went ahead and bought the gun. The price was VERY good, much less than I thought it would be, and less even than some of the more conservative valuations suggested.
I'm going to tear the gun apart, clean it up, and reassemble it. It needs proper early '60s PC Magna stocks, but I'll get those somewhere. Then it will go in my shooting rotation. For reasons offered in the thread below, this may be a rare gun but it is not necessarily a guaranteed high-value collectible because of the uncertainty over possible erasure of a stamp asserting government ownership. I have no doubt that it is a legitimate model 56, as the frame contains metal that would have been milled or polished away if this was a fake based on some other model. It's a 56, period. I will letter it to document what can be said about it from the company records. (The price was so good that I didn't feel the need to demand a letter from the seller before I agreed to take it.)
Thank you all for your comments. Even if you think I didn't listen to you, I did -- then made my decision based on the totality of info available to me.
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UPDATE 2/8/12 : After all the good responses generated by my original post, I have decided to toss the ball back in the owner's court. I will continue to be interested if he letters it and sets an opening price that reflects the gun's condition and lack of government marking, but otherwise I expect I will move on and look for something else.
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(This is where the original post of 2/7/12 started.)
The owner of this Model 56 is willing to sell it and has invited me to make an offer. He declined to put a price on it himself, but knows that the Blue Book values these guns at $7500 NIB down to $2200 in 60% condition.
Apologies for the sort of basic photos and harsh lighting. I used a cell-phone camera under shop lights.
About 15,000 of these were made for the Air Force in 1962-1963, and apparently all but a very few were crushed. SCSW quotes prices that are consistent with the Blue Book scale, which makes me think the Blue Book used SCSW for value assessment, SCSW reports that guns in the 80-90% range have sold for $4000-5000.
I think his gun is about 70%, and I am tempted to offer $2500 for it. It seems to be a righteous Model 56. The only thing at odds with the standard definition is the lack of a U.S. stamp on the smooth backstrap. Perhaps it was once there but was polished off; the backstrap seems to my eyes to have no blue on it at all; it is down to bare steel. But the steel does not seem dished or flatter than it should be, and there are no ghost impressions where a stamp might have been removed.
The serial number is in the right range, the sight and rib configuration is correct, the barrel looks right compared to photos in SWCA. I don't think this is a fake.
The stocks are obviously wrong. The gun would have shipped with diamond magnas, not the post-1968 wood on the gun now.
The gun is mechanically sound -- tight action, no push-off, timing fine, lock-up fine, barrel and chambers slightly frosty but not neglected.
I am not usually a collector of military models, but I am attracted by the rarity of this one. Also, I think snubnose .38s are inherently cool, and adjustable sights just make them cooler.