It's very hard to say.
The pontil scar is right, but the tool used to make the letters isn't the same as the thicker one used on my unique numbered Glasform pieces.
The formation of the letters looks about right, but it also looks really, really like somebody trying to copy his way of writing.
So I can't tell if he was writing badly because of using a tool he wasn't familar with, or of somebody was trying to put a signature one a piece with the right sort of tool mark on it.
I'm not aware of him using a white liner in vases, but then, there's a lot I'm not aware of.
It looks a bit wonky, from your pic showing the inside and rim - John Ditchfield doesn't tend to do unintentional wonky.
The finish on the outside looks about right.