It is a curved brake bridge, the caliper brake fits onto it, so it is in the same plane as the seat stays, it is not a cable stop.

The frame number is only 5543, and it think the 55 is for 1955, 1955 ties in with the little history I know, the 68 year old guy who gave it to me, says it was the first bike he started racing with. Also the Nervex Pro lugs are the old style, which stopped about 57?