The 500 series of oscilloscopes from Tektronix includes:
531 − 10 MHz dual timebase scope, 1954
RM31A − Rack-mount 531
532 − 5 MHz scope, 1955
RM32 − Rack-mount 532
533 − 15 MHz scope, 1958
535 − 10 MHz dual timebase scope, 1954
RM35A − Rack-mount 535
536 − X-Y scope (no trigger/sweep), 1957
537 − storage scope (prototype only)
541 − 30 MHz single timebase scope, 1955
543 − 30 MHz single timebase scope, 1958
544 − 50 MHz single timebase scope, 1964
RM544 − Rack-mount 544
545 − 30 MHz dual timebase scope, 1955
RM545 − Rack-mount 545
945 − Ruggedized 545
546 − 50 MHz dual timebase scope, 1964
RM546 − Rack-mount 546
547 − 50 MHz dual timebase, alternate sweep scope, 1964
RM547 − Rack-mount 547
549 − 30 MHz storage scope, 1966
551 − 27 MHz dual-beam, single timebase scope, 1958
555 − 30 MHz dual-beam, dual timebase scope, 1959
556 − 50 MHz dual-beam, dual timebase scope, 1966
These scopes take letter-series and 1-series plug-ins.