874-X
The General Radio 874-X is an "insertion unit" that allows custom parts or circuits to be built into it.
From the 1965 catalog:
A hollow cylinder fitted with a GR874 Connector at each end and with a sleeve that slides back to provide access to a space about 2 inches long and 9/16 inch in diameter. In this space can be mounted arrangements of small components such as resistors, capacitors, or inductors. The insertion unit can be used as a shielded housing for impedance-matching networks, attenuator pads, VHF transformers, filters, and many other networks. It offers excellent shielding, minimal discontinuity in the line, and convenience.
