I looked at the photos again, this time with book in hand - I am guilty of doing way too much by eye, and off the top of my head, and the older I get the more my accuracy level is beginning to suffer.
The arch ends beneath the middle of the 'scallop' on the right side - that makes it a short (1868) arch, so, given the thicker 'deck', it is basically an 1868 all the way, leaving only the mystery of just why it is not dated.
This could play in favor of Al's theory on the Trials rifle blocks, which he feels were thick, but did not (as far as I can recall) ever say they were not dated. My contention has always been that one of the features under trial would have been the greater degree of opening allowed by the thinner deck of the 1870 block. So, since the 1000+ rifles and 300+ carbines were made at basically the same time in the same facility, under the same authorization, I've always thought they should have the same block.
It will be interesting to see what physical examination turns up. It's good to know that the block will be in caring hands.