My understanding is that it depends on the granularity you want, as explained in the Equipment chapter (note that I'm refering to the former editionof BRP). "Speciality" refers to a skill, while "weapon class" to a family of items.
If you choose a rough granularity, the default rule, then speciality = weapon class and all weapons of a single class are included in the skill, all at the same rating. Note that in old Runequest 2, which is the origin of the BRP, swords were divided into 1H and 2H -and even Shortsword was a class- which explains the difference between Great Sword 5% and 1H swords 15%.
If you choose a more acurate granularity, each speciality is a single weapon (ex shortsword) with its own base rating and specialities do not correspond exactly to weapon classes. Weapons are still gathered in weapon classes as items, not as skills, with the only advantage that using a weapon of the same class is easier (either with the method explained above by @Mugen or at 1/2 rating -GM's choice) than starting at base rating.