Okay, this is the way I understand it......
The master key is needed to program new keys into the ECU/ Alfa Code. Without it you are unlikely to be able to add replacement keys.
The Alfa Code Unit does have a bypass code which was supplied on a credit card style card up until around 1995. AR stopped because owners were leaving them in the car, allowing thieves to bypass the immobiliser. The bypass code since then has been held by National Car Register(?), the number of which you very often see on stickers on the windows (not just Fiat grp cars). If the previous owner transfers a new owners details, then you should have access to all the codes for the car- including immobiliser bypass code, the alarm key transmitter codes (needed to add new keys to the alarm/central locking)as well as the ignition mechanical key code and the alarm on/off mechanical key code.
I found that only some of the codes were supplied for my 155, Mangoletsi were able to find the rest quite quickly even though some years had elapsed since I bought the car from them.