in 1961, an optional locking mechanism consisting of an outer hex nut encasing a captured threaded barrel was added to the 874 line. it can be seen in the photograph of a gr-900 to gr-874 adapter. the locking assembly is not captive and can be backed off the rf connector. the threaded barrel is supplied on each connector. the threaded barrel was withdrawn into the nut on one connector and extended on the other to allow the barrel to engage the nut of both mating connectors. this style of locking mechanism was continued in gr-874's thematic successors; the gr-900 precision 14 mm connector that retains a crenelated hermaphroditic mechanical anti-spin feature to protect the sexless rf interface from rotating and galling when the locking mechanism is tightened, and the fully sexless apc-7 7 mm connector.