If you want to measure it, I think you should measure the shower screen, the basket probably has sloped walls, so that when fitted there's a suitable gap between grinds and shower screen.
As others say, you might be out of luck getting a different pf to fit.
Oh and I just noticed you measured the outside edge, should be inside. So it looks around the 51mm mark, but the shower screen will be more accurate.
If you want to measure it, I think you should measure the shower screen, the basket probably has sloped walls, so that when fitted there's a suitable gap between grinds and shower screen.
As others say, you might be out of luck getting a different pf to fit.
Oh and I just noticed you measured the outside edge, should be inside. So it looks around the 51mm mark, but the shower screen will be more accurate.