It is a feed thru terminal as well as the center assembly. One could bring a two wire cable in one end and leave the other with three wire cable with a hot and switched lead if they wanted. These switches, and similar looking receptacles, lampholders and even some terminal block with a KO in the cover for a pendant cord to exit are common around here on old installs with exposed NM cable from a time before EGC's were being run. There are some updated versions of some these devices that do have a EGC terminal in them as well.
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