Last weekend I just got a 1967 Carlton Giro D'Italia (serial number Ynnnn) and I own several raleigh pro 1971/74 bikes (which are rebadged Giro D'Italias from Carlton). The term "Giro D'Italia" generally refers to bikes with the fastback seat cluster (except a 1st-gen 'thin' seat cluster was offered along with a sloping-crown on the super course from 1967 - 1969). Here are all 3 bikes (super course, Giro D'Italia, BMB) :

https://www.bulgier.net/pics/bike/Ca...68_Carlton.pdf
https://www.bulgier.net/pics/bike/Ca...69_Carlton.pdf

I have never seen a top-tube braze-on cable guide in a pre-1980 Carlton or Raleigh. I think this is something custom, perhaps a custom-order for a certain customer. If that were my bike, i'd probably take it to a framebuilder and get them to fill it with brass the next time the bike needs a repaint and you can stand to burn off the paint ...