On my old (late 1980s) Bianchi frame with a derailleur braze-on, I was unable to fit the derailleur either to the braze-on, or a clamp, to accommodate a 46/30T crankset. (The braze-on was designed for a 52 or 53T chainring, and to put a clamp below it, the derailleur would sit too low.) In frustration I gave away the 46/30T crank and bought an Athena triple (with a 30T granny and a 52T large chainring).

If the braze-on is for a 50T compact crank chainring, it might work.