So I looked at several Olmo frames as you suggested and the seat stays seem to be a reasonable match. However, if you look at a 1975 Olmo they do not have the eyelets for the rear brake cable, but the 1976 model does. The problem with the 1976 model is it now has OLMO brazed on the seat stays. Also, both years have something for the rear derailleur bolt which this does not. Is it possible that it was a lower end Olmo frame that someone put a second shop name on?
As for the idea of the frame being modified, I have been leaning more and more along that line of thought. I think the guides for the derailleur cables were an add on. Although maybe done a little better, the eyelets also seem to be an add on and this would take it back to being similar to a 1975 Olmo frame. The bike was also clearly repainted, so this could all have happened during the repaint. The only other odd piece is it had a Dura Ace stem, with the receded bolt that required a long 6 mm hex wrench to remove. This came after 1980 and all the other pieces were early seventies Campagnolo.
As for it being for the Anglophone world, this might have come in through the Francophone-Italian world in Montreal!!!! The name of the owner offers no clear answer because it was one of the classic half French half English names you run into in Montreal. However, from what I know the owner was a Francophone.
I will check on the wheel/brake question