I found out that they only made 874 .375's up until 1945. So they are quite rare. I know that I have never seen one before. Yes the G&H scope mount attaches to the side of the receiver, so there are three extra holes. From what I have been told, these rifle came with a Lyman #48 peep sight, and the wood was releived at the factory for it to fit. The seller is asking $4,500. Everything is original to the rifle aside from the Griffin & Howe scope mount, and possibly the peep sight. While they did ship with one, I'm not sure if the one on the rifle is a Lyman #48.
These rifles are so scarce that people on another gun forum did not know what they looked like, or what the factory configuration was. One member happened to have a copy of Roger Rules book "The Rifleman's Rifle," and looked up the information for me.
I have noticed that post war pre-64's in .375 have been selling for over $3,000, and they are far more common.
Jared