541
The Type 541 is a 30MHz* oscilloscope that accepts letter-series and 1-series plug-ins. It is exactly like the Type 545 except it does not have the Delayed Sweep (dual timebase) feature. It was introduced in 1955 simultaneously with the 545, and was superseded in 1959 by the 541A, which has more convenient controls.
* 33MHz with a 50MHz plugin such as the Type 1A1.