Slingerland accessory clamp vintage

Description

This clamp, despite its age, is in nearly new condition.   Accepts 5/16" rod (not included) for mounting various accessories.   The patent, 2141818, was filed in 1938 - but I do not know the exact age of this item.   It is, however, at least 50 years old as I have had it in my gear collection since the 60s (and never used it).
It is not designed for a spring - later versions usually used springs.