These boxes were used by several states. I don't recall all of them, but I think most of the boxes retained the same "NG" plate. I think New Jersey changed some of the plates, but at least New York and Pennsylvania kept the National Guard plates like yours. The original inserts were, as you said, for 50-70.
I've seen some for 20 rounds that must be the 45-70 blocks. I bought this one to use while shooting my rifles because it Was 50-70.
Here are some pics of mine. I use it when I shoot my 2nd Allin and Sharps Carbine conversion.
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"Man is not free unless government is limited." -- Ronald Reagan
Neat cartridge box!!!
Fred, thanks for the photos! But, everytime you post photos of your collection I have to change my shirt because of extreme drooling! 8-)