 Adhering manufacturers include AEG (de), Amperex (us), CdL (1921, "French Mazda" brand), CIFTE (fr, "Mazda-Belvu" brand), EdiSwan (uk, "British Mazda" brand), Radiotechnique (fr, "Coprim", "Miniwatt-Dario" and "RTC" brands), Lorenz (de), MBLE(, ) (be, "Adzam" brand), Mullard (uk), Philips (nl, "Miniwatt" brand), RCA (us), RFT(, ) (de), Siemens (de), Telefunken (de), Tesla (cz), Toshiba (ja), Tungsram (hu), Unitra (pl, "Dolam", "Polam" and "Telam" brands) and Valvo(, ) (de).This part dates back to the joint valve code key () negotiated between Philips and Telefunken in 1933–34.Like the North American system the first symbol describes the heater voltage, in this case, a Roman letter rather than a number.