Worth posting again I guess; as far as I know, the only occurrence of other Roman coins as a secondary control mark: I'm not aware of another specimen of this die either, although there may well be others somewhere.
No, it's an as. You can see the mark of value above Janus' head. I always think of it as an aes grave, on the hunch that that would be more "interesting" to a later moneyer, but of course it could as easily be struck bronze.
I believe the coin in Banti page 114 15/21 is the same BUT he lists the device as 'beota' which my complete lack of Italian and about half of the online translators give either as idiot or Boetian. I believe that Boetians were looked down upon by Italians so the two meanings are the same in one sense. However, the Boetian shield was shaped like the two overlapping coins if you allow enough wear to remove the Janus detail. This is a coin that would only be recognized in an EF example unless I miss my guess. A bit of wear would turn that shape into the Boeotian shield. Have you searched for coins that claim a shild as the minor type rather than coins? When you showed this coin before, I wanted one but now I believe what I need will be a worn shield looking stack of coins. That may be a bit of a dream.
De Ruyter's die-study of this issue also lists "shields" as a control type. I've considered whether that may be a misunderstood description of my coin. It may well be, but he illustrates the symbols only with line drawings which doesn't help. I'm generally quite reliant on Banti, but for some reason I checked him for this coin once years ago and never retraced my steps.
Whatever the control mark is---that is one fantastic coin!!! I am so overwhelmed by all the posts (awesome examples everyone!!)....that I don't think I have any others not already posted but I'll throw in a few more anyway.... Denarii of Marcius, Scribonius and Geta: