My understanding is if it has an R, then it should either have a U or an M in front of it. I see the area you are referring to and it does look like a letter R, even kind of looks like another letter was before it but is way more obscured than the letter R. But overall in my opinion, I don't think it is a marking, I think it's just the way the metal is. But the more I look at it, the more it looks like an obscured letter U before the R haha.