Typically mounted on a deck to redirect halyards, sheets and control lines.

This would be a good block for a 22 or 26' Nonsuch mounted on the stern.  Includes backing plate.   $40 new (plus shipping) at Garhauer.   Like new condition for $25 USD with free shipping to anywhere in the US or Canada.

Similar to this model from Garhauer:  https://www.garhauermarine.com/shop/blocks/single-blocks/single-stand-up-blocks/single-stand-up-swivel-block-25-19-us/