Is this a coin from Magnentius or Decentius?

[A. C. P.]

On the coin is what looks like a “X”. Would like to know the RIC number, thank you for helping.
20mm
2,43gr

[maridvnvm]

Decentius. The lower parts of the visible letters can be seen.

[SC]

Yup.  The legend is DN DECENTIVS NOB CAES.

Nice examples of the third series of the VICTORIAE DD AVG ET CAES type - the issue with the small  over the shield. 

It is likely Lugdunum I / LPLG or Arelate I / PAR.  A careful comparison of styles should reveal which mint.

SC

[A. C. P.]

Maridvnvm and Shawn, thank you both very much

[helvetica]

It is not an X, it is a stylized "I".
DN DECENTIVS NOB CAES, bare-headed, cuirassed bust right.
dd nn avg et cae' target='_blank'>VICTORIAE DD NN AVG ET CAE, Two Victories facing one
another, holding between them a wreath inscribed
VOT-V-MVLT-X.
Chi-rho in upper centre, I in lower centre
Mintmark LSLG
RIC VIII Lyons 150; Sear -.

An additional touch, easily overlooked, is that the bow of the wreath forms a chi-rho. Religious overkill ! 
It is scarce and there are no examples on CoinArchives Pro, for example.
As mentioned in my email to you, Tony, I'll add this to wildwinds.