Constantine I The Great AE21 SOLI INVICTO COMITI Sol London RIC 10

Hi.

Does anyone know where to find this coin in the Numista catalouge?

For specific identifications it is always best to provide the weight, diameter, and composition.  Forgive me, but I must say that my first impression of your coin is a cast replica. 

https://en.numista.com/catalogue/index.php?r=&ct=coin&im1=6415f33e985eb.jpg&im2=6415f34c2056f.jpg&tb=y&tc=y&tn=y&tp=y&tt=y&cat=y&ru=&ca=3&no=&v=&i=&b=&d=&ie=&u=&a=&dg=&m=&f=&t=&w=&mt=&g=&se=&c=&wi=&sw=

Looks good to me but as Harry said, weight and diameter would help with authenticity. 

The mint mark is PLN = Londinium (London). In my opinion it's RIC VI 121, the only thing that confuses me is the letters on either side of Sol. They look like T G when it should be T F. It could be that the F merged to look like a G. Here's the example for RIC VI 120 from WildWinds where the F also looks like a G:

Anyway, if I'm right the Numista entry is: N#44954