This is a great thread! Here is what I can contribute so far. The hole in the R is the best way to spot the other dates but Henning claimed to have made 6 reverse dies and 6 obverse dies. Not all of the reverse dies had this hole. The hole is a good diagnostic but not the only one. All Henning nickels I have seen look worn and the surfaces are very rough. Typically, they don't even have full rims. When I check other dates for Henning nickels I first look at the R. Finding the hole is the easiest way to discover one. Next I look for rough almost porous surfaces and a softness in the detail. Even the couple of Hennings I have with full rims have soft details. I have heard other people mention this and say they suspect Henning used worn nickels to make his dies. I agree with this. Another thing I notice on my Hennings is the occasional tiny raised dot or pimple. On the coins with the hole in the R I have seen this inside the M of UNUM. So if I have a coin I suspect of being a Henning I look for these pimples. US Mint nickels will not have them.
His planchets were very close to the correct alloy but not exact. As far as weight, the best I can do is give you the weights of the Henning nickels I have so far. I have 13 nickels dated 1944 with the hole in the R and the weights range from 5.2 - 5.4 grams. I have 12 nickels dated 1944 without the hole in the R and the weights range from 4.7 - 5.1 grams. I also have one nickel dated 1939 with the hole in the R and it weighs 5.3 grams. So it looks like Henning used a group of over weight planchets when he was striking nickels with the hole in the R. But he did strike nickels using planchets of the correct weight. Unfortunately, these were with the reverse without the hole which would make dates other than the 1944 harder to find. I am still studying my nickels trying to find clues to overcome this. So far I only have the clues I listed earlier.
The dates 1939, 1944, 1946, 1947, 1953 are known for Henning nickels. While being interviewed after his arrest Henning claimed to have made six nickel reverse dies and six nickel obverse dies giving the impression there is another Henning date floating around. Some dates suspected in the past for the last die are 1943, 1945 and 1951. But I am not aware of any being confirmed yet.
As far as finding regular mint produced coins on Henning nickel planchets, I don't think this will happen. According to a letter by Superintendent of the Mint Mrs. Rae V. Biester, All of the confiscated planchets (and coins found in the Cooper Creek) were turned over to the US Mint by the Secret Service and the mint melted them down to convert the metal into the proper nickel specifications then produced coinage ingots with it. So Hennings planchets were used by the Mint but in an indirect way.
The historical information I have came out of the book "The Counterfeit 1944 Jefferson nickel
" by Dwight H. Stuckey. This is a 32 page card cover book copyrighted in 1982. It is hard to find (I bought mine in a Kolbe sale).
The only other thing I would like to address is the comment about counterfeits. I completely understand this because of the current problem with the Chinese counterfeits. But my feelings on this are the contemporary counterfeits are a part of our country's monetary history. The historical aspect of our coins and paper money is one of the attractions of coin collecting for me. These counterfeits were made to be used in commerce with our regular coinage. Not to deceive collectors as with the Chinese counterfeits. The fact that Henning was so successful in circulating his nickels and more importantly yet, that they still circulate makes me feel the Henning Nickel deserves a chapter in any US Jefferson nickel
Well I've gone on long enough. Like I said, this is a great thread and let's keep adding more questions and observations to it so we can maybe solve the mystery of the sixth date.