I would not say uncommon. My understanding is that by the SE/30 it was hit or miss. When new molds where made for increased production of cases the signatures where dropped. But molds that may have been modified to slightly new form factors retained signatures, of the most part.

Edit: By the way, increased production also means production started in other countries. So it may be that all the case molds made in Japan don't have signatures while those cases made in Cali do. Not sure if actual cases where made or shipped to Cork.