Following a long dearth, a handful of coins from the Belgian Congo arrived on my desk.  A few weeks later another batch arrived.  This after years of single specimens.
    Coins of that country are very rare in uncirculated, and scarce in grades higher than fine. Corrosion is common.  It's a tropical country, and the climate is not kind to base metal.  So most of these coins were crummy, space fillers while one waits, possibly forever, for a decent example to come along.  I dare say most people's Belgian Congo collections consist primarily or exclusively of fillers.
    As I went through the inventory process with the first batch of coins, my trusty Standard Catalog on my lap, I came across one of those unexplained variety situations.  A couple of dates of 5 and 10 centimes are listed as being struck at both the Brussels and Heaton mints.  Often the Heaton mint would put their famous "H" mark on their coins, but in this case they didn't.  The Standard Catalog shows significant price differentials for the two, but how to tell them apart?  The Standard Catalog does not explain.
    Now, looking at my possible suspects, a pair of 1919 5 centimes, I noticed that one of them was significantly thinner and actually of slightly smaller diameter than the other, so I thought I had something there.  But which was which?
    I put a query out to my 1000 clients and I got three answers.  Though they were slightly different, some points of agreement occurred, and these coincide closely enough with the characteristics of the actual coins that I can now provisionally claim to be able to distinguish the products of the two mints.
    Weight and diameter are NOT distinguishing characteristics.  One guy said that at that time they were being careless with the minor coinage at the mint, I should pay no attention.  The true distinguishing characteristics are in the letter "C" of  "CENTIMES."  Heaton coins have a rounder "C," Brussels "C" is thinner.  The difference is miniscule, a few tenths of a millimeter.  Even with a 12x magnifying glass one could doubt one's conclusion.  But side by side there IS a difference.  The treatment of the "5" is different as well, the Heaton being taller, the Brussels fatter.  But of course that won't help with the 10 centimes.
    Fortunately, there is another diagnostic.  The central feature of the coin is the star around the central hole.  On the 5 centimes the measurement between any first & third point is about 13.9mm on the Heatons.  For the Brussels it's about 14.1mm.  On the 10 centimes the respective measurements are 15 and 16mm.  The star has hairlike rays projecting from it, and the rays from the points end about 0.4mm from the top of the teeth at the rim on the Heatons, versus 0.2mm for the Brussels.
    Ain't that just dandy!
    On the examples pictured here the differences in rims: flat versus wiry, are not significant, or so I've been told.  In my batch the Brussels specimens outnumbered the Heatons by more than three to one.

Click picture for enlargement.