I found this coin on an auction website. I have never seen this type before. According to the seller the diameter is 2.5cm and the weight is 9.45 gm. This would seem to be some kind of medaillon IVLIA AVGVSTA // MATRI CASTR
That's a cistophoric tetradrachm, a denomination that was also struck for Septimius Severus, Caracalla as young Augustus, and Geta as Caesar. All very rare, the Geta unique in ANS from a Bank Leu sale. See BMC pp. 304-6 and pl. 67.1-5. Careless of ocre to misclassify the Domna as a medallion.
Not many Roman women have been awarded the appellation of "to the mother of the camp". I wonder if she got that as the wife of Septimius or the mother of Caracalla. Also the medal is supposed to be of silver but in the OP's image I see either a laminar or a plated medallion. Do you think it ever circulated as currency?
From what I remember reading, she was given the name "Mother of the Camp" due to her traveling with Septimius Severus while he was on his campaigns.
A Russian collector friend of mine wrote me this (my translation): This is a tridrachm of Julia Domna, minted most likely by Caesarea of Cappadocia or another city in Asia Minor. The inscription on the coin - Matri Castr is stands for mater castrorum - mother of camps, apparently in honour of the fact that Julia Domna was involved in providing the legionaries with the necessary supplies. This would also explain the bundles of corn on the reverse.
A better specimen from the same dies, without the large pit on portrait and deposits on rev., brought £1800 = $2771 = 2117 euros, plus buyer's fee, in Morton & Eden 64, 5 June 2013, lot 120.
MATRI stands for matri, not for mater, first is dative, the second is nominative. Often there are altars who celebrate an augusta as matri castrorvm because she proceeded to rebuild or renewal a camp or barracks. The pic below show an altar in Ostia placed in honour of Julia domna because made some works in the vigiles’ barracks. As you can see engravers errors were present on marbles also, here you can notice how they wrote MATRI AVGVST, and then they added a small “I” above. They were so frequent not only in officials marbles. Below always from the vigiles’ barracks another altar issued for gordianus with a similar error.
The use of a letter "K" for the initial C of castrorum makes me think the engraver's first language was not Latin.
A Julia Domna "Matri Castrorum" denarius Julia Domna, wife of Septimius Severus. AR Denarius (16mm; 3.15 gm; 6h). Struck 196 AD. Obv: IVLIA AVGVSTA, draped bust right. Rev: MATRI CASTRORVM (Mother of the Camps), Domna standing left, sacrificing from patera held in right hand, holding box in left hand; two standards left. RIC IV 567; BMCRE 57; RSC 134.
Not properly, is well documented that the use of k instead of the c started at the end of II century and largely continued during the III century, for example in Rome has been found several inscriptions of genius kastrorum peregrinorum, and in general many inscription of the III century with the use of the K.
