These pulleys are frequently found in mid 20th century single-glazed residential windows.
Like the originals, these are zinc plated steel. The round bottom sheave is intended to be used with #8 sash cord. Sash chain is not recommended. These economical pulleys have serrated “fins” at the top and bottom of the housing are designed to “dig-in” and secure the pulley where they are mortised through the jamb.