...and filed neatly to shape...

...until it was a loose sliding fit in the blade slot.

For solder to take well the steel needs to be degreased with something like cellulose thinners, and then abraded with 120 grit silicon carbide paper.

You can then refit to the blade and mark up where the slot will be.

Also mark clearly where the line is 4mm above the existing cap iron adjuster slot.

First a word about SOLDER. I use high silver solder from Maplin Electronics shop. It is 5% silver, 95% tin, 0% lead with no flux core. This melts a little hotter than electricians multicore, but still well below the temperature that would affect the temper of the steel.
You can use tinmans liquid acid flux, but I found in storage the fumes from even a sealed container rusted every steel item in the cupboard, so I now use plumbers "Telux" grease flux from B and Q.

Spread flux over the join area, and snip off a couple of mm of solder to lay by the joint. This will indicate when the joint is hot enough but not too hot as to burn the flux or harm the steel.