"some kind of" and "some kinds of" | WordReference Forums
There are two kinds of some: the one that means "(a) certain unspecified", always pronounced like the word sum, and the one that works like an indefinite article for plural or noncount nouns. In "It is some kind of pen", "kind of pen" is singular count, so this some is of the first kind.
grammaticality - Some kind of + (singular or plural ...
english.stackexchange.com › questions › 111666This happens to be a fairly specific use of the word "some," which you find often, but not always, with synonyms of "kind," thus: some kind, some sort, some type, some version. In all these cases, "some" means "one of the items in this group, but I don't know which one." "Some sort of car" means "it's a car, but I don't know which."