The Engine this came off of, came out of a C body. Even though it don't have the "187" stamp, I assumed it was still that pan. Ma did wacky stuff as we all know. I'd love a good answer on this myself. Not trying to insult anyone's intellect.
Your pan clearly has the same notch as the 187, it just didn't show up good in your first few pictures. When you shot the inside of the pan, it's clearly visible.