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Looks to me to be a rust spot. When the bearing was removed it allowed the rusted area to expand (which it couldn't do before due to the crank stopping it) this then bloomed out to look like it is now.Check all the others for similar damage, you're probably lucky as it's extremely rare for this to happen in my experience. You could just replace this bearing with another green but I'd strongly advise buying a pair of green bearings to replace those on that journal. Don't order new ones until you've checked the others very carefully. Your lucky, green is about the only colours it's still easy to find. Yellows and blacks are rare and browns are starting to go the same way. If your stuck finding new bearings contact me as I have quite a few NOS ones. I started to collect them many years ago and it's coming to the point where I'll sell them all and cash in. As soon as I've finished building the engine on my last restoration I'll be selling a lot of stuff.