Help Identifying An Ancient (With A Twist)

OK guys, I recently started reading up on the history of vending machines (I live a sad life).

Anyways the first ever vending machine on recorded history was developed by Hero of Alexandria. It was essentially a locked box with a slot to drop in a coin. The coin would hit a lever/balance beam and push one side of the beam up. This beam was attached to a string that would pull up a plug and let holy water come out. Like a Rube Goldstein device or what ever they are called. This was done to prevent people from buying holy water then taking too much and being greedy with the stuff.

So naturally I want the coin used in the first vending machine.

So we know Hero of Alexandria was born and lived between 10-70 in, well, Alexandria. During this time Egypt was under Roman control.

According to the University College London "In Egypt Roman coinage was based until AD 296 on the Ptolemaic system. The first issues are some copper based coins. Regular production started in AD 20 with a tetradrachma. Most of the coins in Egypt were struck at the main mint in Alexandria. As with many Roman provincial coins, the obverse usually depicts the portrait of the emperor or members of his family. The legends are in Greek. The reverses are varied and offer many types only known from Egypt. Some depict gods such as Sarapis and Isis. Personifications of the virtues such as Nile on a crocodile or Nike ('Victory' in Greek) holding a palm branch are also common. The representations on coins seldom reflect political or other current events. Issues for the Egyptian nomes can be found honouring the administrative provinces and are typical of the second century AD in the Antonine Period (AD 138 - 180)."

Great, so the we are probably looking at a tetradrachma with an ALE mint mark between 20-60? Can anyone here help me narrow down the coin some more?

This is probably one of the strangers questions to be asked in the ancient sub-forum, eh?