Originally.... nothing. Once tin was discovered they tried that. It's okay to use, but you have to be very careful so that you don't anneal the base of the blade. With a thin guard like the false one (disc) I could have brazed it with silver solder, but I had the JB already mixed up for the thick guard (royal PIA to keep evenly heated enough to braze) so I used it on this disc also.
Also with old swords and knives it was the pommel that held the whole thing together.... get the tang red hot, put the pommel on and rivet the tang so none of it would go anywhere.