#BlameMoo
This line:

should be:

else if (selection == SelectionPlace.HANDSTONE && inventory.getStackInSlot(TEQuern.SLOT_HANDSTONE).isEmpty() && inventory.isItemValid(SLOT_HANDSTONE, heldStack))

Also, i think this issue can be further enhanced by:

• Instead of using ore dict to check handstone validity, use instanceof ItemHandstone
• Delegate the client-only drawing functions to the new ItemHandstone class
• Also, would be awesome to delegate the handle and input hitbox positions there
• Lastly, if we want to open up more control of the quern's features to addons (like, if a better handstone rotates/do recipes faster) add everything needed to this new class.

I disagree with the change to using ItemHandstone as a render delegate. We use ore dict as it's the obvious customization option for the least effort (on the pack maker side).

What should not happen, is the handstone should not be drawn in world using the actual item. That should be a constant render, using any handstone.

Addons using the quern should very likely just add their own quern (or, if they're adding an actual "better handstone", just make a better grinder. This is not a case to sacrifice simple customization for addon compatibility.