The main compression tube is steel. There's no die cast feel to it at all. Most complex trigger in airguns (there might be some crazy Euro trigger more complex but in the real world of airguns, nothing else compares.)
Back in the '80s, I was picking up 600s for less than $20 on a regular basis. Would turn around and sell them to guys at work for $100. Several got mad because they were spending too much money on Powerlets and pellets.
Note: they made left-hand grips that were made available at the Service Stations, so, they can be found. I know some guys have used the left-hand set to make the grip without the thumb rest.