Many are according to say that Gillette used Probak's handle after taking over the company. Also, their base plate are different from the others old type, there's grooves on each side.
Very thorough and concise info as always Mr PorterYes, those corner notches would have meant that the razor wouldn't work with Gillette's old three-hole blades, and would have only worked with their NEW blades that had the corner knock-outs. That would make this something of a Goodwill variant, but where they've not bothered to change the cap studs. It would also put its manufacture no earlier than the release of the NEW blades, which was what precipitated the Auto-Strop merger that brought the Probak line into the Gillette fold.