I don't know about all 401A's but ours is almost all metal. The only non metal part in it is the gear on the back of the hand wheel. And that's a fiber gear rather than plastic. I have seen them listed for many different machines though and the replacements are plastic.
I suspect the 500 will have some plastic in it, but we don't have one, so I won't swear to it.
The 538 is back up and running. A bit noisier than normal but it's working. It does need the cam drive gear replaced and I'll get to that as soon as I can.
Joe