This article (I don't know the authenticity of the writer) mentions that the signatures on some items has disappeared over time because it was done in enamel (I presume cold painted ?)
https://leverreetlecristal.wordpress.com/2014/01/22/andre-delatte-1887-1953-et-ses-galinettes-france/
This may be true and I don't know what they were signed with however I do have a few pieces from the period 1930s with the black schwarzelot type enamel used on them and having just checked them that doesn't seem to me as though it would come off, even the signatures although I have seen signatures chipped. So perhaps they used something else to sign ADelatte Nancy? and perhaps also the Lorrain signatures which also look a bit watery for want of a better description?
Correction to what I wrote above - I've just checked another one of my other pieces by Henri Quenvil and the signature is indeed rubbed off in the middle portion. It's v good quality and fired enamel decoration on the vase and no chance of that rubbing off but the signature is, leaving the start and the end of the sig but rubbed in the middle.
So it seems that is the case.