Constantine  I Magnus (307-337 A.D.) - AE3 - 18.90mm, 2.84g
Obverse - CONSTAN-TINVS AVG
Constantine, his laureate and cuirassed bust right.
Reverse - VIRT-EXERC
In exergue. dot TS dot Γ dot 
X-shaped plan of Roman camp, with Sol standing left over the center,
his right hand raised, holding globe in left and chlamys across left shoulder.
R.A.G. Carson suggests in "Coins of the Roman Empire", p.166,
"ït is perhaps better to be regarded as a bird's eye view of a monumental
stepped-base surmounted by figure of Sol".
Wayne Sayles suggests it depicts "a type of defenseive barrier".
The images above depict the coin from my collection.  The one on the very top was taken by me using a flatbed scanner. The 2nd
image was taken by me using a digital camera. The third image was taken by the person who sold to coin to me.

Added below for lllustration purposes is the image of a signifficantly better specimen of this coin, which was offered in May 2001 by
Numsimatik Lanz, München, as lot #987 in their auction with a pre-sale estimate of $700.
That specimen is listed at Wildwinds.com here:   Text   Image
In Rasiel Suarez' excellent coin auction database Coryssa you can find it with the details from its listing at the Lanz auction here.
Minted at the 3rd officina (Γ) in Thessalonica (Saloniki, Greece) in 319 A.D.

Reference:
Bruun, P.M., "Roman Imperial Coinage", Volume 7 (RIC), Thessalonica, p507, nr 66.
Failmezger, V., "Roman Bronze Coins From Paganism To Christianity 294-364 A.D." (Failmezger), p34, nr 288CI.
Voetter, Otto, "Die Münzen der römischen Kaiser, Kaiserinnen und Caesaren von Diocletianus bis Romulus, Katalog der
hinterlassenen Sammlung und Aufzeichnung des Herrn Paul Gerin
" (Voetter-Gerin), Thessalonica, Constantinus Magnus, p338, #4, nr. 11.
Sear, D.R., "Roman Coins and their Values" (RCV Millennium Edition), Volume 4, p484, nr. 16305.
Suarez, R., "ERIC II - The Encyclopedia of Roman Imperial Coins" (ERIC II), p875, nr. 2893 (B68, O4, R30, T126, M17, Exe: .TS.Γ.)
Suarez, R., ERIC II-Digital edition-Part IV,  p748?,  nr. 2893 (B68, O4, R30, T126, M17, Exe: .TS.Γ.)
Cohen, H, "Descriptions Historique des Monnaies frappees sous l'Empire Romain" (C), Volume 6, p306, nr. 658.
Kampmann, U., "Die Münzen der römischen Kaiserzeit" (K, 2e Auflage), p423, 136.197.

Quality: Fine
Scarcity:  R5 (= unique)

My notes: At the time of publication of RIC Volume 7, in 1966, Sutherland and Carson could only find a single specimen of this
coin from officina Gamma and Delta each in a major collection, the one from off. Gamma being located in Paris at the time.
Since at least one specimen from each officina also existed in Paul Gerin's collection, as described by Otto Voetter in 1921, this
already seems rather silly. The coin is not as rare as suggested by the authors of RIC. Still, when I saw this (albeit rather worn
specimen), come up at at auction in the Netherlands for next to no money, I immediately recognized it for what it was and
couldn't resist adding this intrigueing coin to my collection.
My personal opinon on what is depicted on the reverse alligns mostly with that of  R.A.G. Carson: I think there may have been
a giant monument with a statue of Sol on top of it that was erected in Thessalonica (as this coin was only minted there), but
which is now lost to history. We may never know for sure...
Depicted above is an excerpt from page 86 of the book "Die Spätrömische Kupferprägung" by Guido Bruck (published in 1961)
which lists not only all of the existing legends and bust types for this coin, but also the frequency with which specimens of these coins
were found in Vienna. Please note that it contains 5 specimens for Crispus (RIC 69) followed by 4 specimens for Constantine I
(RIC 66 and 67), and 1 specimen each for Licinius I (RIC 68) and the Caesars Constantine II (RIC 71) and Licinius II (RIC 70).