It looks like just an ID mark, maybe a factory or a brand. Probably UK in origin. It's not much different than an "X", dots or a line on a bitters or something, it really doesn't mean very much and just about as impossible to trace.
Sand_pontil and Eric. I just posted a blurp about bottles made in past molds, where the mold seams were wiped out by turning the glass while it was in the mold that was lined with a mixed paste painted in the mold. I can't help that this bottle might have also been turned. RED M.
I must have made my comment above on a different picture. What I am seeing on this bottom picture is what I think is a makers mark that was used to identify the shop that made the bottle. This means that the bottle could not be turned in a paste mold unless the body of the bottle was tapered. Sorry about that - I don't remember the situation. RED Matthews