Lubrimont sold it to me and Dean Russell confirmed the marking be right for my ex-GT Jr. ..... I'm only going what they sold and confirmed .....head side is between 38-39mm using my measuring devices. Dean volunteered the info without prompting him when I was in search of one and being in Detroit he should know as they've trickled past him over the bridge. My '72 Super 1.3 doesn't have it on there. Don't have a headWell if that was the symbol matching that for a 1300 head I might agree. Its not.
What is the horizontal diameter of the manifold's head side and does that suit a the stock 1300 head port dimension. Can you post the other markings as I have done at the thread starter post.