I'm unfamiliar with St. Euphemia holding the Creed in her right hand
The Great Martyr St Euphemia, the All-Praised, was martyred at Chalcedon early in the 4th century. In 451, the Council of Chalcedon was convened in the cathedral under her patronage to address the beliefs of the Monophysites.
When the Council couldn't come to agreement, it was decided to open the Saint's sarcophagus and place two scrolls therein - one in support of each party. The sarcophagus was sealed and guarded and the Council Fathers prayed for three days that they would receive a sign. When the sarcophagus was reopened, the Monophysite document was at the feet of St Euphemia, the other was clasped in her hand - or hands - causing the Council to condemn the Monophysite belief.
In doing some more searching, I find some recountings that state it was in her right hand - others in both hands - and I notice that a line drawing of St Euphemia (on the website of a GO school in Australia which bears her name) also shows it in her left hand. I'm now definitely leaning toward Deacon David's impression that it is her.
Many years,
Neil