OK, re-checked the record. The Russians say the best sounding Moscow-made 6H8C are with shiny perforated anodes, made from carbonised nickel: Moscow factory (МЭЛЗ), metal base, late Fifties. 1578 is an after-manufacturing testing quality assurance stamp, to certify less than 1% difference of transconductance between the triodes, and some other characteristics. These were intented for control panels of the nuclear plants, and the earliest Soviet "computer". 1578 quality stamp was awarded to tubes of different factories and different structures, according to my information. My MELZ tubes have neither ink stamps on the base nor 1578 etching on glass, just factory stamp and date etched on glass. I have seen large quantities of these tubes with ink stamp on metal base with the date as late as October 1979, which is funny, as according to my records MELZ switched 6H8C from metal base to black carbolite base in the 1960ties.