The Tektronix 013-028 is a "ring box" accessory used for calibrating the Type N sampling plug-in.
The description of the 013-028 in the Type N manual is as follows:
Ring-Box --- Designed to ring at frequencies of 1000 mc,
500 mc, 200 mc, and 100 mc (periods of 1, 2, 5, and 10 nano-
seconds) when excited by a fast-rising pulse. It is used in
calibrating the NANOSEC/CM switch on the Type N Sampling
Part No. 013-028 ....................... Price on request