I use acsearch for identifying coins quite often. And whenever I see a reference for any auction that was held by Numismatics Paris, they always put a rarity with a number on their listings in the same format. So that system must be from somewhere. Example below. And you need google translate for the French quite often. Like I never did check what R3 means etc. But it is someones scale.
POSTUMUS
Type : Sesterce
Date : 261
Mint name / Town : Trèves
Metal : bronze
Diameter : 30 mm
Orientation dies : 6 h.
Weight : 16,33 g.
Rarity : R3
Obverse legend : IMP C PO-STVMVS P F AVG
from https://www.acsearch.info/search.html?id=10618062
POSTUMUS
Type : Sesterce
Date : 261
Mint name / Town : Trèves
Metal : bronze
Diameter : 30 mm
Orientation dies : 6 h.
Weight : 16,33 g.
Rarity : R3
Obverse legend : IMP C PO-STVMVS P F AVG
from https://www.acsearch.info/search.html?id=10618062


















