 The mica spacers used in "SQ" and "PQ" types did not possess sharp protrusions which could flake off and become loose inside the bulb, possibly lodging between the grids and thus changing the characteristics of the device.Some types, particularly the E80F, E88CC and E90CC, had a constricted section of bulb to firmly hold specially shaped flakeless mica spacers.For examples see below, starting at DC
Later special-quality tubes had not base and function swapped but were assigned a 4-digit number, such as ECC2000 or ED8000, the first digit of which again denoting the base