I have several pre-1900 Parkers with these lines. In fact, the process of drop-forging or "bulldozing" the billet into the desired shape of the raw frame block has caused a very visible uniform curve of these lines along the side of the frame up into the standing breech. If I could figure out how to post pictures here I would do it. If someone cares to give me their email address I will send him pictures and he can post them. I have never thought they were anything other than imperfections in the billet and the fact that they curve with the shape of the frame leaves no doubt of their origin.

dsromig@aol.com
Dean