Martin,
I think the coinage of A. Pius as
Caesar is about as
scarce as might be expected for an issue lasting a little over four months.
Diana is probably the most unusual
rev. type, and occurs only on
denarii.
I say a little about the coinage of Antoninus as
Caesar in the
thread "Some
rare Antonine
sestertii with ordinary
types", now on p. 2 of Classical
Numismatics. As I say there, Philip
Hill, followed by David
Sear in
his new
Roman Coins and Their Values, wanted to eliminate the coinage of Antoninus as
Caesar and reattribute it to the period of Antoninus' supposed struggle with the Senate over Hadrian's
consecration, but this is certainly an error!
For Antoninus' coinage as new
Augustus in 138, also very interesting historically and including many
rarities, see my answer to Varangian in the
thread "An interesting bronze
medallion of
Hadrian",
Roman Coins, p. 4.
You've dated your coin incorrectly: you meant "25 Feb. - 10 July 138".