No malice at all taken.  The receiver photo doesn't show it well, but there is faint outline of a thumbprint right over the worn area on the receiver.  I had a gunsmith go over it when I bought it all those years ago and he declared it safe to shoot.  He thought initially what you did, but there was no heat discoloration or indications of welding inside the receiver.  Could the thumbprint be corrosion from heavy use, with either sweaty or bloody hands?  My own left thumb falls right into that area if I was loading the carbine.  I do have a photo of the cartouche, but it is very faint. I will include it anyway to see if you can make anything out.  It looked like 1897 to me.

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