TL;DR; Please add key version at the same level of key generator.
I've built a userscript that converts JSON Feed into an readable HTML.
I'm adding support for ActivityStreams.
I can determine if a file is of JSON Feed by looking into json_file.version, but not so in GNU social (StatusNet) ActivityStream feeds, unless I look into key generator, but then again, some other software (Mastodon et al.) might choose or have a plugin to deploy ActivityStream and they could use a different key.