Didn't know Zeus supplied frame builders with fittings back then. Every day's a schoolday! However, the top tube cable stops for bare rear brake cable might help date it? When were they in vogue?
Just noticed the head badge is chromed as well. Did Carlton do this - or might it have gone in to the chroming tanks attached to the frame if it was rechromed (or just chromed)?
My 50's Flyer doesn't have top tube cable stops for rear brake.
Badge looks like its been chromed over. Good spot.
It has Italian-style lugs, so may help on which model it is. These became more common in the mid-late 60's afaik.