I have in my possession a circular sash router and rebate router of identical construction to the branded Preston models, but with no indication of a maker whatsoever.
My guess is that these were either made by Preston and supplied unbranded to one or more of the big wholesalers, or else were copies after the rights ran out (or maybe even before). There are of course loads of examples (esp for planes and drills) of established designs of cast metal tools being very widely copied.
The old Buck and Hickman and Melhuish's catalogues show these tools without naming Preston as the maker, although adjacent illustrations show the distinctive patterns and logo that Preston used.
However, with the Preston brand so often making tools more collectable, and giving the price a shove upwards, it could be just too tempting for an eBay seller to make the assumption that if it looks like a Preston, it is a Preston, and should sell like one.
below is what I wrote up on my site about a quirk router, all the blades where Preston by one this was marked Gleave Manchester, I would have thouht Preston did make them for tool dealers but not marked?
I still have the picture on my site if you view even though it has sold, sorry not allowed to put link on here, but if you do look you will see most like a Preston tool.
Preston's Improved Circular Quirk or Grooving Router.
although not stamped by Preston I am certain this is made by Preston exactly the same model as their No 1388, it comes with three fences and 5 cutters 3 by Preston one by Gleaves and one not marked, much of the japanning is intact.
Price Reserved Inclusive of postage to mainland U.K.