Recently I was asked:
"How do I differentiate between the two London Mint T-F issues. For example this SOLI INVICTO COMITI coin of Constantine I.
Obverse: CONSTANTINVS P AVG Laureate and cuirassed right
Reverse: SOLI INVIC-TO COMITI Sol standing left, chlamys draped behind left shoulder
T-F across fields and in exergue PLN
Now, this coin with identical descriptions is listed in RIC VI as London 127 (rare), year 310 AD (ref "VG 45")and RIC VII London 93 (R1) year 316-317 AD. (ref only as "London")
So I wonder, do you know what the visible differences are between these two ?"
So, how do you tell? The answer is weight, die diameter and style. If you had one of each together, the differences stand out a mile. If you just have one on their own, it can be a little more difficult. Size, by which I mean die diameter – the distance from one side of the beaded perimeter to the other – is key. The earlier coin is 21mm, the later one 19mm. My examples of the earlier mintmark weigh between 3.25g and 5.6g. The later issue usually weighs in at between 2.5g to 3.8g, so there is some overlap but wear and flan thickness can usually explain this. Finally, and the most difficult to explain is style – sometimes I cannot tell the difference and much prefer to go by die diameter and weight as supporting evidence. Having several examples of each issue, all I need to do is compare a coin against my “large” issue and my “small” issue.