Exactly... the last three or four digits could be the same as the original flight number. You would still need to know the operating carrier, as the second poster notes, but with co-location and common check-in desks even that is becoming less relevant.

There could be one bucket of six digit numbers reserved for common *A use, eg

UA 852 = CO 110852 = NH 110852 = TG 110852 = US 110852 = *A 110852

The identifier is just that - an identifier. The actual flight number and airline are the original.