My Deville case has a lug broken off in the same manner.
The Case is an American made type with no sea horse, just a flat blank space for engraving.
It appears there was a steel stem welded to the case and the gold plated lug was either cast around the stem or bonded to it in some way.
On trying to silver solder it the entire stem melted away instantly, though little heat was used, so it couldn't have been solid gold. Gold takes a lot more heat to melt than solder does.
Its the only flaw of these watches, the lugs are far to flimsy. In looking for a replacement I've run across dozens of deville cases with one or more lugs broken off.