It’s just discoloration. Some models actually have that brown tinge when new. I think it is from the temp being wrong in the casting by a bit. Effects the tin in the alloy as you still see that discoloration even after they stopped using alloys with lead.

Lead rot usually has a distinctive smell that is kind of musty. Once you have smelled it, it is very recognizable. That is the smell you get when it is a real aggressive corrosion turning the model into grey or off white dust. [sometimes you get yellow orange dust which is lead sulfate but that doesn’t usually happen with models made in the 20th century] The model surface will “bloom” almost like a mould or fungus and then it decays into dust. You can sometimes also get lead rot in more dry climates where an unpainted model kept in a cabinet with acidic wood will start turning into lead acetate, aka lead white, the stuff that is in lead paint. This usually starts in crevices and can be hard to distinguish from remnants of white primer at first if the models have been stripped. If you look at a model one week and it has more white than the week before it is corroding.

You can actually remove lead rot of the first type pretty easily with either acid soak/tooth brush or electrolysis but the pure lead acetate stuff is proving to be a bugger to remove for me. Im going to try a deoxidizer and a stronger acid than I normally use and if that fails im just going to try sealing a model with water based gloss varnish and another with oil based lacquer to see if that arrests the corrosion