I can see that it’s a 6mm shaft but I can’t see what material the shaft is. Is the material known?
Having used the 19:1 and 50:1, it seems to me to be mild steel.
I could be wrong, though, so an answer from our helpful Pololu-ians would be interesting to me, too.
The shafts on our 37D gearmotors are made from stainless steel.