The vast majority of "Bulls eye" windows are modern fakes.
The first glass was made as a cylinder and then opened out, later blown glass was blown as a sphere and then spub into a large disk. Panes of glass were then cut from this. The bit in the middle, the bulls eye was discarded and recycled but was sometimes used as the cheapest of the cheap
There's an article on old windows here:
http://www.periodproperty.co.uk/article020.htm
You can still get glass like this for restoration from places like Pearsons they call it drawn antique
http://www.pearsonsglass.com/stainedglass.asp