I am enquiring about how this part is measured is it diagonally across or from hole to hole on outside, and was wondering what model pump this Diaphragm fits?

I have asked the warehouse to re-measure this for you and they have come back to me saying: 16.5" between opposite lugs and 11.5" between one lug and the one next to it. I have also asked and this diaphragm is the suit the Wickham 900 pumps.