Yes, they were - although the opinion that this c/m may have been part of the unofficial die has been fielded: British or Gallic "Limes Falsa" Contemporary, underweight, regional copy of Æ As of Agrippa, c. 40-50 AD. 27.5mm, 8.43g, 5h. With TIAV "countermark" by Claudius applied to allow the coin to circulate as valid, possibly designed as a feature of the reverse die.
Justin Lee, pointed out that counter-stamping was also done in Syria, which triggered my memory. About 8 years ago I won an odd looking AE as from a CNG auction struck in the reign of Claudius, see photo below. This coin was previously in the collection of Richard McAlee & is illustrated in his book "The Coins of Roman Antioch", page 132, #250(c)/1. The coin is 25 mm, & weighs a hefty 18.45 gm. Two coins on that same page have strong counter-marks, showing the same practice was going on in Antioch, Syria too. McAlee makes an interesting note in his book "that none of the Claudius portraits in his lifetime from Antioch are realistic. The Hellenistic tendency to idealize the ruler is still in evidence, accentuated by Claudius's ugliness, as seen on his Roman portraits."
That's a very nice Claudius, Al! Here's a heavily worn one with a countermark as you mention... Was this the type of countermark in McAlee's book? It is incuse Minerva/Athena standing right with spear and shield (Howgego 245). The c/m is believed to be struck during Domitian's reign (after ~40 years of circulation) due to his strong interest in Minerva, and for the same reason the other c/m were struck in Roman Britain, approval of coinage that could otherwise be in question. It's quite the contrast to the typographic c/m used for the Brits.
Justin Lee, That's a great counter-mark on your coin, however, it's unlike the two McAlee illustrates. Oddly he doesn't describe the counter-marks on either coin & the photos don't look like the quality that would scan accurately. #250(d)/2 appears like "1E", & #250(d)/3 looks like "NEB", with the B below the NE. He does state that the counter-mark "IB" was used to indicate years 11 & 12 for Claudius since no aes coins were made in those years.
That's got me really intrigued! McAlee's book is one that is at the top of my list, but is just out of my price range.
I wrote a page on British Claudian imitations: http://augustuscoins.com/ed/imit/imitclaudius.html After illustrating many Claudian imitations, near the bottom are a couple of non-British Claudian imitations with countermarks: 27 mm. 6:30. 7.05 grams BON countermark. This coin may be from the Balkans, not Britain. [As in French, BON = good] Kraay, in Essays Mattingly, suggests the BON countermark may have been applied under Nerva or Trajan. This coin is very worn, compatible with having circulated a long time. BMC 164, a sestertius, and 184, a sestertius, both imitations, have this BON countermark, but there are no photos. Banti & Simonetti 61/9 has BON countermark = Hunter I, 17.67 (7.87 g) with different countermark die. 32 mm. 6:00. 13.47 grams Struck imitation of the SPES AVGVSTA type sestertius. (Prototype next below.) Countermarked "DV" which is not listed in BMC I and which is coming to be recognized as from the Balkans. [Revalued as a dupondius, as one of the OP coins] Obverse legend: ...CLAVDIVS CAESAR AVG ..." Reverse legend weak, but visible: SPES AVGVSTA . AE35. Sestertius Official prototype: Sestertius of Claudius. SPES AVGVSTA, Spes standing left, holding flower. This type was copied in Britain, but not as commonly as the Minerva as. The site on imitations has no illustrations of British imitations of this type--the previous imitation is from the Balkans. For reference works on imitations, see: http://augustuscoins.com/ed/imit/imitationrefs.html
Thank you @Valentinian, @randygeki and @eparch. I am in the process of upgrading my web page. The current iteration can be viewed at ……… http://jp29.org/000counter.htm ……… I am in the process of incorporating updated information to include new or upgraded coin photos (when I am able to take them) - all this may take some time. I will post the updated page link here. Again, thank you everyone.