Hi,
I just had a look at other Harcuba signatures and it seems possible that you are right. I have no idea about his non-engraved work, but as he was clearly an artistic guy, there is no reason why he wouldn't produce designs away from engraving. I know he worked in the States for some years, perhaps he has an archive in the USA, or people he worked with are still around to ask? Best of luck,
Robert (bOBA)