If its a genuine Mullard it should have an etched code on the glass for tubes made after 1950 or so. This is a two part code, the first letter-number combo dictates the build structure and the 2nd character set, the plant (factory) and the date code. There are instances where the code is hard to read, but it should always be there. Earlier tubes have the gray smoked coating in the glass, later ones not.
I also read Brent Jesse's comments about Brown/Black base, He is very reputable. So I don't quite understand? The Brown Bases can take more heat build up they were used a lot in guitar amps, because of that they could be used vertically or horizontal. After reading the ambiguity on Brown/Black I called a few knowledgeable people I know and they said they have seen genuine Black Base Mullard's. I'm doing a crash course in tubes, just attempting to get good solid info to avoid any issues or problems...
I also read Brent Jesse's comments about Brown/Black base, He is very reputable. So I don't quite understand? The Brown Bases can take more heat build up they were used a lot in guitar amps, because of that they could be used vertically or horizontal. After reading the ambiguity on Brown/Black I called a few knowledgeable people I know and they said they have seen genuine Black Base Mullard's. I'm doing a crash course in tubes, just attempting to get good solid info to avoid any issues or problems...