250
The Tektronix Model 250 and its rack-mount version, the R250, are instruments introduced in 1968 that extend the number of control lines of a model 240 Programmer.
Having more lines allows, for example, a 240 to control a R116 or R293 pulse generator as the stimulus for a device under test, while simultaneously controlling a 230 and a 568 for measurement of the response.