The Tektronix Type 3S7 is a sampling plug-in for 560-series scopes such as the 561 and 564. The 3S7 appeared in the 1971 through 1973 catalogs. Used with the 3T7, a TDR system is formed. The 3S7/3T7 are seen on the front cover of the document Tektronix TDR Measurement Concepts. Like the 1S2, the connectors on the 3S7/3T7 are 50-Ohm GR-874. A short coaxial cable carries the pulse from the front panel output of the 3T7 to the front panel input of the 3S7, where it passes through the sampling gate and out to the cable or network under test.