Obvious what the tool does (makes straight lines in cement finishing), but my online searches for PETKO only turn up Petko Industries of Los Angeles that made home door keys in the 1960's. I haven't been able to find anything about a Petko hand tool line. Any old cement workers in this group? I don't need another sub, sub, sub group of vintage hand tools that only work cement... but... it was only a quarter at a yard sale, and I've jumped down worse rabbit holes.   

I found two wood chisels with the Petko brand. It seems that the company was based in southern CA but the chisels were made in England. An internet search provided no additional information.
-Don

Hello, Moparthug . You are right about Petko Industries .I found this page . Thanks to Internet Archive for the catalog photo!