Is Sear 882 (off the top of my head, the large module SCL counterstamps, counterstamped on large pre 530s folles) proof that folles from 498-530s still circulated in Hadrian's time? Were these oldies counterstamped in Constantinople and lugged over there, or were these existing coins in Sicily? Were a great number of these imported into Italy during Justinian's invasion, perhaps explaining the paucity of marked Rome and Ravenna 40 nummi? How about elsewhere? When did these types of coins go out?
Here's an example of an Anastasius follis overstruck during the Sasanian invasion of Syria, c. 611-625: This certainly suggests that at least a few older coins were still circulating in Syria. And here's a Maurice Constantinople overstruck on an older coin, probably Justinian (I haven't made a concerted effort yet to ID it, may not be possible): Oh, and one of the Sicilian countermark types you're asking about: I'm not sure about the explanation for these.
Wow, very interesting, esp. the Sassanian! Sadly, I have no big SCL issue; only some small ones. I'm working on finding a good one. I bought a Seleucia placeholder until I can find a better one, too.
Oops, finally noticed my typo: Hadrian = Heraclius. Whether the large module coins were already there in Sicily or if they were old stock from Constantinople, the coins were hanging about somewhere. Interesting how they stuck around.
These overstruck coins come in all degrees of success. My favorites are those with the most clear undertype remaining. Having the new obverse struck on the old reverse can make separating the strikes easier. My second is less clear on the reverse but had added appeal to my eyes and fingers from the chainsaw sharp edge. The Byzantines were fortunate not to have had pockets for this coin would have cut its way out and been lost.