This question was interesting, and in honor of the nature of this question, the alternate title for this post is "Reference Question of the Week - X/X/X."
One of my brothers came across some old Roman coins, and asked me if I could help identify them. The cleanest one he had looked like this (front and back):
He had found out about a book called Eric: The Encyclopedia of Roman Imperial Coins which should be a fairly comprehensive list, and wanted to know if my library had a copy. We don't, and according to WorldCat, it's not widely available at all. It is published by Dirty Old Books (which is the best possible name for a coin book publisher), and I found the full-text available as a PDF download [ZIP, 65MB!] from their website.
After it downloaded and I started flipping through it looking at the pictures, I was overwhelmed to see how many different Roman coins there were - but I guess that's what you get when you rule the earth for centuries.
So, I decided to try a different tactic. I went to Google Image search, and started searching on phrases that described the back of the coin - "roman coin two people tower," ancient coin two soldiers," etc. I started switching between both Images search and Web search, thinking the text descriptions in the Web search might give me more keywords, and that's exactly what happened with "coin soldiers standing tower."
On that search, the third return was titled Constantine, Roman Imperial Coins reference at WildWinds.com and had in the description "...two soldiers standing front, holding spears, heads turned inward at two standards between them..." which sounded promising. On that page I did a Ctrl+f for "two soldiers," and seemed to definitely be getting close. I also noticed that the words "GLORIA EXERCITVS" were printed on a lot of the coins. The writing on my brother's coin was too faint to read, but now that I knew what to look for, I could pick out the letters.
So I tried an image search for "GLORIA EXERCITVS" and hit the mother load.
Clicking on a few of those lead to http://www.romancoin.info which had a lot of information. I couldn't find the exact face on the front of the coin, but by scrolling down a bit, I found the exact coin back under the "Reverses" section of "Part 1 – The Era of Constantine the Great, his Sons and Rivals (320 – 337 A.D.)." So that seems to limit the coin's era and it also linked to a spreadsheet [XLS, 412KB] of all the coins that use that have that design on the reverse.
It didn't make much sense to me, but I sent all of this to my brother hoping that between the websites, spreadsheet, ERIC pdf and him using a magnifying glass to get more detail from the coin, he'd be able to identify it exactly.
But then again, narrowing it down to the reign of Constantine the Great might be enough. Unfortunately, it seems like these coins are fairly common and not very valuable. Still, pretty neat - I love my job.