RO Membrane Size Help

[Cal]

Hi, I've purchased a second hand RO/DI system and after some tds readings I've figured I need new filters. I have an issue with finding a RO Membrane to fit. There is no info on the original filter other than the brand name and '200g' so after googling I found out it's 2812 although when I run a tape on the diameter it says 3inches.. anyway so I purchased replacement 2812-200 filters to find out they are smaller than the original. So now I have ordered 3012-400 and it's still smaller than the original in size, it's no different to the 2812-200... Am I missing something? I've not actually unpacked the filters as may need to return them but I'm certain they are small than the original.