I have a silver or platinum eternity ring. Inside the band the only marking is what looks like a long H on it's side inside a box. Does anyone know what this means?

The letter, whether it is upper case or lower case, and the shape surrounding it, combine to indicate the year the piece was formed. Each May, a different assay master was appointed and held office through the following April. To match a precious metal piece to the assay master who oversaw its assessment, a system of revolving letters was used, one per assay master's one-year term. Twenty letters of the alphabet were used; J, V, W, X, Y, and Z were not included. Each letter was used in turn, five times per century. Each time the letter came into rotation it was varied in style or in the presentation of its surrounding shield

More than likely it is the makers mark/ artisans mark.