All Winthrop products include a permanent mark, this is what to look for:
Winthrop butts have the Winthrop logo and patent number etched into the metal.
Each roller guide and roller top display the name “WINTHROP” on the black roller caps and also a part number laser etched on the bottom side of the metal portion.
Winthrop stripper guides also have the WT icon milled into the frame.