Last coin looks to me an imitative issue with christogram degenerated from 
to 
Bendall notes this
type as no. 12 on p. 150 and
his comment to it is as follows (p. 154-155):
"Until the 1990's this
type was extremely
rare but a number of specimens have recently appeared in the two
hoards closing with coins of
Maurice Tiberius and Phocas.
This
type is somewhat enigmatic. The
monogram formed by the letters TX on the
reverse does not represent the name of a city.
Hahn has suggested that it may represent a
monogram of the word
Tyche and be a partner to
type 10 which he thought represented Synkletos, both coins struck at the same time, celebrating both the senate and the
good fortune of
Constantinople. This is very unlikely since there is a certain amount of evidence to show that
type 12 is later than
type 10. The fact that the first
hoard mentioned above contained coins of
types 8 and 12 as well as
forgeries of tremisses of
Justinian I is not of importance since the coins of both silver
types were crude, small and corroded while tremisses of
Justinian I were extremely common and could have been forged even after
Justinian I's death. Most specimens of
type 12 have only been found in two recent
hoards indicating that it was circulating both slightly before and after 602. Before these
hoards the
type was almost as
rare as
types 9-11. The
type is
still very
rare and obviously not found in sufficient quantities as to form a significant
part of the currency as does
type 8c.
However, the photocopy of the
Lanz polaroid indicates that
type 12 was circulating in small quantities while the siliquae of
Justinian I were
still quite unworn and
type 8c comprised the
bulk of the silver currency. Since precious metal coins at this time tended to be hoarded and did not circulate from hand to hand regularly as earlier
Roman denarii had done, it is perhaps not surprising to find siliquae of Justinian so well preserved many years after they were issued. Silver coins of the sixth and seventh centuries are often found in
poor condition but this is usually due to
poor striking and subsequent corrosion but seldom through wear as a result of circulation. The form of the
reverse, a
monogram, is not unlike the
Chi-Rho monogram on the later « miliarenses » of
Tiberius Constantine and
Maurice Tiberius and might therefore confirm a date of с 580 for the introduction of this
type".
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