The Type 535 is a 10MHz* oscilloscope that accepts letter-series and 1-series plug-ins. There are two timebases, and triggering is done with a Schmitt trigger made of a 6U8 tube (in early instruments) or a 6DJ8 tube (in late instruments). The 535 is functionally the same as a 545, but with a simpler, non-distributed vertical amplifier. Early 535s came in brown, square-cornered cabinets; in 1956 the cabinet became blue and round-cornered. Early 535s had selenium rectifiers. The 535 has a thermal cutoff.
Type 535, the single-timebase Type 531, and the first four plugins, Types 53A, 53B, 53C, and 53D, were introduced in August 1954, advertised as '...the "convertibles" of the Tektronix line'. Types 541 and 545 came out in March 1955.
In 1959, Types 531, 535, 541, and 545 were superseded by Types 531A, 535A, 541A, and 545A, which have more convenient controls. In addition, Types 531A and 535A have slightly faster (15MHz vs 11MHz) vertical amplifiers. There is a rackmount version of the 535A, the R35A. Starting in 1963, the 535B was made with BNC connectors for the external trigger inputs. Prior to 1963, the external trigger inputs used UHF connectors. 1970 was the last year that any version of the 535 was sold. In 1971, the 54x series was still available, but nothing from the 53x series.