Roy's correct: a sewing machine needle.
Not that I do not have more proper scribers, for normal use: quite a few of them, TTTT.
On some (rare) occasions I need to trace lines much thinner than those I can get with them.
Marcello
Nice job there, Marcello. Only improvement that I could think of is to have a square or something on the shank to stop it rolling away, but I see you have a block to stand it in, so in your case maybe not needed.