I think I see that hole in the dropout also. That is for the Campagnolo Sportman (1961) derailleur. Which would not likely have been used on this bike, the dropout is simply ready for it. There are a lot more of those dropouts than there ever were Sportman derailleurs. And the hole was even there before the derailleur was finally ready to go. Dating indicators are not perfect.

That's a Nuovo Record derailleur. Introduced 1967, too new for this bike. Earliest ones were undated.

There are extant Model Bs with all original kit that have some surprising parts. Mafac, Weinmann, Altenburger brakes all happened as well as Universal. Altenburger derailleurs got used. You have some leeway. Even if you want to be perfectly period correct you have leeway. If you want to ride it rather than display it try to be respectful but do what you want. Your bike. Velostuf has original Gaslo silk bar tape for you. Silk tape is nice.

It's a fabulous bike. Get it together and get it on the road and see how you like it. Decide later if you need correct REG bottle cages (handlebar mount?) or if your pump peg is too early or too late.