I suspect that this casting is made from 'spelter', an indeterminate term which means different things to different people in different times, but loosely interpreted as 'brass trimmings, scrap and swarf' chucked unceremoniously in a crucible and heated to melting point….

….Unfortunately, during this melting of brass junk there was plenty of oppportunity to add contaminants that make use of modern solders problematic….antimony, is common and can have detrimental affects, as can a variety of base metals and stuff like mercury and oily muck……

I have come up against 'spelter' clock frames and cases, notably those extravagant looking french clocks with partial greek columns and trophies of laurel leaves etc….vile in both appearance and content! It's common to find that they can't be repaired except by drilling and plugging. Sometimes the 'metal' will not even allow threading it's so poor in quality..