Hondo6

Depends on the grip area OD of those "old silver" drop handlebars.

ISO spec for drop bar grip area OD today is 23.8mm (15/16"), and I'm pretty sure that predates the introduction of STI's. I'm thus pretty sure all STIs are designed to fit that OD.

However, some older steel drop bars used a grip OD of 22.2mm (7/8"). I don't know if the clamps on all STIs will contract enough to mount securely on bars with the smaller OD. A difference of 1.6mm isn't that much, but depending on the clamp design it might be just enough to prevent proper fit. Haven't tried it personally.

Older French bars (25mm clamp) used a grip area of 23.5mm, so I'd guess those would very likely work. I'd certainly expect STI clamps to accept a difference of -0.3mm from nominal, as that's less than -0.012" smaller than the ISO spec.

You could try it if you already have the bars and are dead-set on using STI shifters (and will thus be getting a set of STIs anyway). But you may or may not end up needing a new set of bars in that case.

Shims of thickness around 0.7-0.75mm placed between the clamp and a old bar with 22.2mm grip area would also theoretically work. Keeping them in place during mounting might prove difficult, though.

More info on handlebar dimensions can be found here.