Is the Tetradrachma you posted from Sear's article? If not, where is it from? Although it does look like copper plated with silver, I would also think it looks like a modern copy and not an authentic original. My resaon for saying this is two fold: 1) it looks very round while coins of this era tended to be a little more oblong and 2) the imperfections in the flan and owl look lioke casting bubbles.
On another note, there is some question about whether these silver plated Tets were official issues or contemporary counterfeits. I don't remeber all the issue but there was a very good articel in The Celator that discussed it. I think the artice was in the last three years but I don't have it handy.