Regular Heliopan filter adapters allow you to mount a larger filter on a smaller lens.
You read the item description in the way that the first number is the female thread (where the filter gets screwed into) and the second one is the male thread where the adapter gets screwed into lens.
I you have a lens with a diameter of 55mm, and another one with 52mm, your best bet is to get all filters in 55mm and then buy an adapter 55/52E.
If the first number is smaller than the second you try to mount a smaller filter to a lerger lens. This can lead to vignetting.
Vignetting can also occur with wide angle lenses or if an adapted filter is combined with a sunshade.
All items marked with "Special Order Item" will be custom manufactured for you and cannot be returned. Manufacturing takes usually about 7 to 10 days.