The Leica m6 coating leaves the base low and swells due to partial loss of adhesion, causing the surface to appear as a round convex deformation. The bubble may contain steam, other gases or crystals. In general, this phenomenon is due to the bubbles generated by the surface coating, and the gas may contain organic substances, microorganisms, and even corrosive chemical components. In the process of coating and film formation, the bubbles are flattened, and with the increasing pressure, the bubbles of the cousin’s coating are finally squeezed out.
In fact, the cause of this phenomenon in Leica M6 is likely to be that Leica m6 period reduced some processes, because m6 sales exceeded expectations. In order to increase production, the factory reduced some process processes that had no obvious effect. Of course, this is only a guess for i50mm.