If we ignore the pre-'71 covers which are a different shape, the oil tank covers are interchangeable. There are of course fibreglass and steel versions.
The fitting systems differ on the battery side cover. Those intended for use with the steel air filter case have a bracket with spring clip which fixes to a dzus fastener on the frame gusset. These covers have a tool tray on the inside and two grommetted holes at the rear.
Mk1A and Mk2A 850s which had the black plastic air filter have no bracket but have a visible pressed indent on the lower right hand side for use with a plastic dzus. On the Mk111 850, the pressing returned to its original shape and had a welded-on tag to keep the lower edge in place, retaining a plastic dzus for fixing to the air box at the top.
It is quite easy to make a simple bent bracket to use a Mk111 cover without the air box. I have an early cover on my Mk111 but I have a 750 airbox and battery tray as well. I think you'd have to remove the top bracket from an early cover if you wanted to use it with a late air box.