Antoninianus - Maximianus (CONCORDIA MILITVM)

I got such a coin during an exchange.  I'm not an expert on such coins, and the letter S is confusing. There is also no book in my city's library where I can look at it in more detail.

Looking at it with a magnifying glass, I don't think that this coin could be made of silver, or even covered with silver. At first, I couldn't see from the picture that it still shines very pink. Copper? Bronze?

Now that I have this coin - then maybe someone will be able to identify what it is. Initially, SirFabs classified this coin under this link N#306638. Here is also a picture.

Thank you very much if anyone takes the trouble to guess something about this coin.

 A lot of similar coins, with different mint letters. 

The S is for Thrace, Heraclea. 

https://www.cgbfr.com/maximien-hercule-aurelianus-spl,v43_0697,a.html 

 Details from there 

 Lastly 

· OBVERSE
Obverse legend : IMP C M A MAXIMIANVS AVG.
Obverse description: Radiate, draped and cuirassed bust of Maximian Hercules right, 

 seen from three-quarters back (A2).
Obverse translation: “Emperor Cæsar Marcus Aurelius Maximianus Augustus”, 

(The Emperor Cæsar Marcus Aurelius Maximian Augustus).

· REVERSE
Reverse legend : CONCORDIA MI-LITVM./ S// XXI..
Reverse description: Maximian Hercules and Jupiter standing face to face; 

 Maximian standing on the left, turned to the right, dressed in military attire, 

 holding a parazonium in his left hand, and receiving a globe from Jupiter, naked, 

 standing on the right, turned to the left, with his cloak over his left shoulder and 

 leaning on a scepter in his left hand.
Reverse translation: “Concordia Militum”, (The Concord of Soldiers). 

· CATALOGUE 

C.54 - RIC.595 

The lack of silver color is also fine. This is an antoninianus, which stared as a silver coin, but during the 3rd century, the silver content was gradually decreased. By this time, it was about 5% or less.