If I understand correctly, it is a little strong to call them "fakes". "Rebranding" was and is routine business practice. For example, not every tube was made by every company. The E91H, a switching tube used in early computers, was made only by Amperex, and therefore, every E91H made was an Amperex. This tube was extensively rebranded by many companies, including Telefunken, Mullard and IBM. And then you have companies like Dumont. Dumant was not in the business of making tubes. They were in the business of buying tubes from the lowest bidder, putting their name on them and selling them. In other cases, a manufacturing company may have received an order for more tubes than they could produce, so they reached out to other manufacturers to meet demand. So we see Amperex tubes made by Mullard, RCA and Mazda. We see Telefunkens made by Amperex, Sylvania and Siemens. We see Siemens made by Mullard, Toshiba and Telefunken. We see GE made by Sylvania, RCA and Mullard. And the list goes on and one. Again, this was and still is routine business practice.
And after all, the 6DJ8 was designed for use as a low-noise amplifier in VHF and UHF TV tuners, and in these circuits, the brand made no difference: a 6DJ8 made by Sylvania was a plugin replacement for a 6DJ8 made by Telefunken.