The ones in treadles are very heavy on my digestion, much more so than potatoes.
If I have understood it correctly, a few of the early 600 numbers were all metal geared. The 600 and 603 are suppose to be metal. The 600 numbers were Singer's transition face, and it seems you have to check each machine for the plastic gears. The 626 and others have also been reported to be all metal, but some have the plastic gears. I have been looking into this too, since I have been interested in the bobbin functions of the touch and sew series.