It's a little bit Norman Stuart-Clarke-ish too, I tend to agree with both John and David, the marks look suspiciously not of the same period as the glass, and are really rather meaningless to anybody who knows how IoWSG marked things.
I do have two pieces marked "IoW glass England", these came from the archives of the studio and were sold at a much later date than production. They were marked (I was told by Tim) just before leaving the studio. But they both have "glass England" written after the IoW, and neither have any initials on.