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General Radio 1650-A
Coax insertion unit
Genrad 874-X Insertion Unit

Available from 1954 to 1978+

Catalog History
 Document Year Page
Catalog N 1954 139
Catalog O 1956 59
Catalog P 1959 55
Catalog Q 1961 43
Catalog R 1963 72
Catalog S 1965 92
Catalog T 1968 144
Catalog U 1970 159
Catalog73 1973 250
Catalog78 1978 101
(All manuals in PDF format unless noted otherwise)

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.