sorry Margaret........I'd forgotten for a moment that you are new, and I should have explained more clearly what I meant
Somehow you seem to have escaped the usual beginners 'I'm new please be gentle' tag.
In the U.K., from approximatley the 1840's until about the middle of the twentieth century, manufacturers of pressed glass often registered their new designs, with the Patent Office, as a means of protecting their copyright of a particular design and as a means by which to try and prevent copying of their items by other factories. A sequence of Nos., eventually running into six figures, and as Anne says preceded by Reg. No. or Rd. No. formed part of the glass mould, and thus when pessing a piece those details are transferred to the article, in relief. Unfortunately, it doesn't hold true all of the time, and glass is found, frequently, without these details just to make life difficult. I believe that a lot of Davidson Pearline is marked, but no doubt a lot isn't. Within a distinctive range such as Davidson Yellow Pearline, I would imagine most shapes are well documented, and it is in the area of clear glass that difficulties are greater.
Hope I've now put that right, and I bet Margaret knew all of this anyway.