I extend this ticket with two proposals.

First is to use r/1837 and determine ourselves, the hard-coded way, rarity
multipliers for the item grades.

Second is to add a "rarity" field at item spec, optional, which influences the odds.

It usually requires some more patching because in the end, we are deciding the
final drop, not the drop chances. And this can be a little problematic depending
on how we do this decision. (ie. this may require more code lines)