Hiya Cobber,
Good on ARB for that actually.
You can get them from a whole heap of manuf's. Some will stamp a rating on them most won't. My understanding is that the ones with ratings have been calculated using the steel materials specification and the distance from the hole to the plate edge to arrive at a number... or... they can equally just pluck 'em outa their butt.
There is no Standard so nothing they can be held to anyway. Same as a Snatch strap, if an 8,000 jobbie snaps at 4,000 it is just tough.
But you raise an interestiong question in... So what is the point of replacing an unrated hook with an unrated plate?