Unidentified Prussian ? Medal

Here's a brass or yellowy bronze medallion in my collection.
39mm diameter. Weight 21.32g,
Obverse: Bewigged man in uniform with decoration on left breast FREDERICUS BORUSSOR REX

Reverse: 3 standing figures: left, a female holding scales (Justice ?), centre, a male holding stick and sceptre, right, a female holding a spear. NIL ORTUM TALE. In exergue:
IURISC PRUDENTISS
IMP SUM PRINC OPT
MDCCLVIII

The medal is worn, which I think is unusual. I've tentatively graded it VF/AVF,

I assume this is a Prussian medal of Frederick the Great
I've found it via Google as item 15793 in 'Mes Loisirs, Amusemens Numismatiques by Clément Wenceslas de Renesse-Breidbach see
http://books.google.co.uk/books?id=...2nil&f=false

1748 was the end of the War of the Austrian Succession whereby Prussia gained Silesia. Does this medallion relate to this ?

Anything you can tell me including its value will be of interest.