Eric,
Museums do sometimes sell or trade away duplicates. Or occasionally museum coins can be stolen and so find their way onto the public market!
There is an innocent explanation, however, which could well apply to your coin.
Strack might have mixed up
his casts. He visited the
Vienna collection in 1927 and 1930; but in 1930 he also visited a very rich private
collection of
Roman coins in
Vienna, the
Trau collection, which was dispersed five years later, in 1935, in a famous
auction.
Now,
Strack was apparently trying to collect plaster casts of every Eastern
denarius of
Hadrian that he came across. If he was making
his own impressions and casts, he might have made impressions of the
Eastern denarii in both the national
collection and the
Trau collection during that same
Vienna trip in 1930. Your coin might be a Trau coin, but
Strack got
his batches mixed up and labeled
his cast of your coin as coming from the national
collection! Such a mix-up would of course be much less likely if
Strack instead asked museums and collectors to make casts for him and send them to him by mail. I don't know which of these methods
Strack used.
If your coin indeed belonged to Trau, it is not illustrated or individually described in the sale
catalogue, so presumably formed
part of lots 1174-6 in the sale: 50, 50, and 52
denarii of
Hadrian respectively, in "
good or
very good" condition, which probably equates to "
fine or
very fine" today.