Well, let's call it a 1967 or 1968. Legnano serial numbers, if they ever meant anything, went largely undeciphered. Around 1966 the decades long method of stamping them into the top of the seat tube appears to have been abandoned in favor of placing them on the underside of the BB. In addition the old serial numbers were changed from the two letter/four digit format to a different system. Your "275" can probably be termed a "serial number". I have two Legnanos from, I believe, 1966 and 1969-70, with BB serial numbers of "36" and "232R", respectively.
As for the missing name "Legnano" since it was painted on, someone probably scraped it off.