miriam,
On automotive distributors I normally use a center punch or a scribe to put a dot or a scratch in the parts. I haven't had to do this yet with sewing machines, although I tried with my rusted 99K. I found those parts VERY hard and even the center punch didn't scratch them.
So, at this point I'd be tempted to get a carbide tipped tool of some kind that would make a mark.
Joe