The Tektronix SA501 is a signature analyzer plug-in for the TM500 system. (Also known as the 067-1090-00.)
It requires the P6458 probe for control inputs and the P6405 probe for the data input. The dedicated 4-channel control probe P6458 can be substituted by the similar 8+1 channel P6451 probe without increase in functionality.
Note that the P6407 probe connected to the SA501 is not compatible and can be damaged when the SA501 is powered.