FN's from Isreal are supposedly all 12(3) as well. Don't believe it. The G Series name comes from the US sites when they describe the early FN's that came in after the GCA in 1968 when the FN was described as an auto rifle if it had the safety sear. (yes there is the German G1 series as well, most ended up in Turkish service and are now torched up into kits) The laws here are different, find out what it was registered as previously.
The C1/AussieL1 share the same lower as the C2/L2 with the exception of 3 internal parts, but that is all that makes them different from 12(3) and 12(5). There is no "A" stamped on the lowers unless they were made into C2/L2's though the cut was there until the end of production.
The rifle is worth quite a bit more that an over-priced 1A1 at $400. Depending on what accessories come with it, I would pay up to $1000 for an all matching, perfect condition "g" series. If by the grace of God it was an actual German G1, I would pay quite a bit more as they are very scarce in their original form and issue condition. The last ones being issued to the West German Border Police well after the army switched to the G3.
YMMV
Gene