I think this is just a fascinating debate, so I'd like to provide further background on the subject. First of all, my illustration of the Pompeii elephant figurine is after a painting of the object found in Peter Connolly's Greece and Rome at War. Initially Murray and I thought we should feature a speculative illustration of Caesar's elephant crashing through the Thames. Polyaenus states that the animal was armored and with a tower, yet there is very little evidence to suggest the specific Roman method for equipping their elephants. This Pompeii figure could have been made to represent an elephant from any number of ancient armies, but still I thought it was safest for the illustration to reference a depiction of a war elephant found in an early imperial Roman context. Connolly's book doesn't provide any detail on the find, but I imagine its provenance doesn't extend beyond "found at Pompeii."
The coin and the Polyaenus strategem about Caesar were discussed in a 1959 article, C.E. Stevens. "Julius Caesar's Elephant," History Today, Vol IX. Coincidentally, the dating of this coin was being discussed on Forum Ancient Coins just as we were working on this article in July. You can access the Stevens article through JSTOR. Most university libraries in the US have a subscription to this service. Or, if you contact me I'll be happy to email a jpg of the article. (It's only 2 pages).
John Kistler's 2007 book, War Elephants also takes on this little controversy. My local branch didn't carry this book, but I got a hold of it though interlibrary loan. It's an easy book to find, but you can also view the pages on amazon.com. Ramon Jiménez's 1996 book, Caesar Against The Celts speaks of the elephant issue, but the scholarship of that book has been criticized.
I do recommend reading War Elephants. I borrowed it to research for this illustration, and kept it to read afterwards. The book includes the use of war elephants into the modern era, but I didn't read past his account of ancient times.