Date of manufacture is the criteria, so I'm guessing that all GA's were T4. I'm not sure where the turning point was but I have a couple '77/'78 and they are T4 forends with the center trip lever release. The three '80's guns here are all T4 of course, with the right side lever release and I have no certain date for that transition. Prolly enough P-gun folks here to get the data to fix that if someone doesn't already know. The good news is that the hangers do look really different even if you don't have the forend.