Yes that is correct, older bars from 70's 80's bikes were typically 38cm to 40cm c to c width, occasionally there are 42cm bars too; I think the idea was that narrower bars made you more aerodynamic, although now the idea is that you match the width to your shoulders, or even wider to allow you to breathe properly or whatever.
The clamp diameter for drop bars depends on the origin, any east Asian bikes would typically be 25.4mm anything Italian or lots of other road bike bars would be 26mm. 90% of older bars will be either of these diameters, however there are some oddball ones like Cinelli which made 26.4mm diameter bars, Raleigh had their own 23.8mm upright bars etc.
Almost all modern bars are now oversize 31.8mm (~1-1/4") which is more or less standardised, however you can still get older 25.4mm or 26mm clamp size bars from makers like Nitto or Velo-orange quite easily; choice is somewhat limited compared to oversize. Typically yo
Be sure to match the clamp size of the stem and handlebar, don't try to fit a 26mm bar into a 25.4mm stem even if you can pry the stem open. You can do the reverse however with shims, although you can have problems with slippage.
