IMHO, if it's like that going along each of the pull-ups then it is meant to be there since it looks purposeful to me. The black speckles and collection of black around some of the bubbles within the same area looks like its supposed to be there since it doesn't have a random look.
If it was just like that only in one small area that might be different.
Although having said that I am conscious that I have seen similar random effects on Nazeing glass, particularly from the 1930's, that has half-hearted attempts at creating a different effect where it does not come off, but seen enough that you realise they were trying something out - but I digress as that's not Gray-Stan!