The stretch stitches on the T&S didn't start until the 700 series. They are achieved using the T&S white cams.
There are some T&S machines that have metal gears. They include the 600, 601 and 603. There are a few models between the 600 and the 626, but are not common. As I understand it, they have metal gears. Some have the "auto-reel" bobbin mechanism that is pretty "fluky". Some of the auto-reels work and some don't.
I've heard that there are some 626 machines that have metal gears. I've not seen one. However, most have plastic gears. Everything after the 626, including the 648 have plastic gears.
Like I said earlier, I've trashed 6 T&S machines that I bought at auction or people gave me. All had broken gears. You can tell, as when you turn the handwheel, the bobbin doesn't turn.
The one that I've had since it was new still works.
bkay