appearance of burn on 7308 Amperex tube

Hello

I am fairly new to tubes and have been doing some tube rolling of late in my preamp, a Blue Circle BC 21.1. It uses 2 tubes from the 7308/6922/6DJ8 family. Only recently when I was swapping out the 7308 Amperexes (white label USA)for some 6922 PQ's USA white label did I notice a 'burn' or the appearance of something like that on the shoulder of one of the 7308 tubes. I don't notice any sonic degradation and both channels seem OK at the moment. Any ideas? Thanks all for your help. Dave