I always have to look it up to remember, but "Clean Easy" is correct. Only offered in black (actually a greyish black and especially so when etched some) the porcelain coating was standard on dutch ovens only beginning in the early 1930's and continuing for some time; maybe into the 50's but I am not sure. It was also offered on the smooth and hammered dutch ovens and was optionally offered on the No.8 sized only hinged skillet covers in the late 30's. So, in the 1930's the porcelain was common and probably was standard on all d.o.'s.
As far as cleaning goes I would recommend that you do not use lye on the porcelain surface as my experiences "seem" to indicate that the lye etches the surface of the porcelain making it rougher and lighter grey. Not worth taking a chance. Fortunately the covers will float upside down in a lye bath if you gently lower the cover on top of the bath carefully. No problem getting lye on the porcelain but don't let it sit for long without washing off. Otherwise I see not special precautions you would need to use. Maybe lye or oven cleaner can be used for a short time to clean off any baked on grease from the porcelain but, again, may not be worth the possible risk to try that. Oh, and don't wire brush the porcelain.
Steve