In case it hasn't been mentioned - I can't recall - I believe the half-size stamp suggests it was either a team bike or a custom build. I once owned a 59.5 myself, now in the hands of another CR listmember.
Also, Fred Radnor wrote regarding the brakes on the ebay bike: "The central bolt is removed from the brake, shortened and brazed into the frame. Then the calipers are reassembled in the usual manner."
This was also done on some Bianchis of the era, including the bike with which Felice Gimondi won the 1973 World Championship Road Race. You can see the way this was done with the front brake (note the undrilled crown) here:
I've seen the same modification on a Bianchi owned by a listmember and probably built for either a pro or a top-level amateur - I forget which.
Lancaster, Pennsylvania, USA