You know I was only talking about the MONETAE type, not IICOS reverses in general, right? In any case, your coin is very far from common! I haven't taken the time to do a really thorough search, I was relying more on @maridvnum's assertion that he hadn't seen another one like mine & his. (Until the OP, that is.) Here's what I have: II COS reverse: Maridvnum's has the PEPT obverse die & a different reverse die, all the rest share the same obverse die as yours. Besides Barry's, there's a Rauch sale (https://www.acsearch.info/search.html?id=4880941) and the BM has one (https://www.britishmuseum.org/collection/object/C_1997-1203-45). Both of these have the same dies as Barry's. So 5 coins total, 2 obverse dies, 3 reverse dies. AVG reverse: 3 total (now). All share the same obverse die, Maridvnum's has a different reverse die. The BM has one with a IIC obverse (https://www.britishmuseum.org/collection/object/C_1997-1203-78) and there's this Naumann example with a COII obverse (https://www.acsearch.info/search.html?id=1943840). Those two have different reverse dies. I can't infer anything about order of striking from all that, but maybe you can! I defer to your expertise.
I don't really send my coins to NGC anymore, the way I like to display them, I just use DIY slabs that have the self foam inserts (pvc-free of course). I just don't see any reason to send to NGC unless I want to resell coins that way or if I get a really expensive Gold coin, but I'm less likely to. If that really is some dirt deposit on the reverse of your Septimius and you plan to remove it, break open your slab and let the coin rest in distilled water for 1-3 days and then use a very light non abrasive tool to "lighty brush" the looser dirty away. It does look to be just patina though and if it is, I would definitely just leave everything alone. Your NGC slabs aren't airtight long term anyways so the coins would eventually get some nice toning to blend with that patina mark.
@Severus Alexander - the Naumann coin is mis-attributed. That is a COS I. I have had a couple of these I think I have 3 AVG II Cs from various dies I do have a couple of oddities too. I have a reverse die link for BONETAE (sic) between the AVG issue and the AVG II C issue I don't have enough information yet to add any analysis to the relative chronology however.
I forgot to add the PEPT - II COS And here is an even more strange oddity AVG II C - FORTVAE(sic) REDVCI, Moneta/Aequitas standing left, holding scales in right hand, cornucopiae in left
If you want to get dirt off a silver coin without damaging the surfaces with abrasions, use pure 100% acetone. Not nail polish remover. dont use water and a rag because abrasions will occur on the surfaces. i think your coin is nice as is though. You could always upgrade in the future if you’d like.
I still consider my oddest oddity to be my IICOS reverse Domna. Sorry it is not a Moneta. I don't think this was an accident or a mule. I just believe that the coins were produced by people not firm in the concepts that ruled on Rome mint coins.