Heliopan Adapter Ring 58/49E

The Heliopan 58-49mm Step-down Ring is a step-down ring permitting the use of 58mm threaded filters on a 49mm threaded lens. Constructed of matte black brass, Heliopan step-up rings are highly durable and longer-lasting when compared to aluminum rings. Efficient solution for use of one size of filters with various sized lenses. Matte black brass lessens chance of glare. Rugged construction for ease of use. 

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.