I seem to recall that the figure
S stamped in series on these blades was similar in meaning to those bite marks (hogsback/ eyelash marks) on blades from Genoa...as a mark of excellence. Please see
http://www.fightingarts.com/reading/article.php?id=183 and
http://mandarinmansion.com/antique-vietnamese-arms
Is the round guard missing?