1950s Colnago "La Freccia" (Arrow).

1950s  Colnago La Freccia, (ARROW), with Colnago FM Headset.  Down tube and seat tube have "1957 Campionato  Lombardia Dilettanti" decals.
*Bottom Bracket:  Italian 68 mm. 
*Dropouts are 1st Generation Campagnolo Gransport with 4 mm adjuster screws.  These dropouts were produced only  in about 1950-51.  This may have been because the oversized 4 mm-threaded shoulders were vulnerable to breakage and the Derailleur hanger had an impractical  4 o'clock position  stop, as illustrated in a set in the last foto.  Unfortunately, the rare derailleur hanger  was removed and the bike was fitted with a  Simplex TDF derailleur.
In 1952, Campagnolo updated the  dropouts with smaller 3 mm threads.   Campy also introduced the industry standard 7 o'clock  position derailleur hanger stop--still found on 21st century frames.
Top pics show complete bike with Universal Extra Brakes, Campagnolo GS Hubs, Simplex Derailleurs, Brooks B-17, Ambrosio Champion Bars Stem, Sheffield Sprint pedals and Magistroni cranks with  Simplex rings.