URLs are primary keys.
They are primary keys describing a context.
I think having two ways to describe the same context is a bad idea (although, certainly possible).
If I know only one key describes the context, then if I have two keys that are different I know they don't describe the same context.
I think that's useful.
That is to say it would be useful if everyone saw the world the way I do...
That's not to say that different contexts couldn't necessarily return the same data, of course. ;)
If you want the context for an entity, and you want to support anything more than 'current' then the context needs to be aware of time.
Version is usually the best way to describe time.