Maximian Herculius, AE follis, London mint. 303 AD.

IMP C MAXIMIANVS PF AVG, laureate, cuirassed bust left,
holding spear over shoulder, and shield.
GENIO POPV-LI ROMANI, Genius standing left, modius on head, 
naked except for chlamys over left shoulder, holding patera 
and cornucopiae.
No mintmark.

RIC VI London 29b var (obv. legend), extremely rare.

The seller described the coin as being one of Galerius,
but I believe that the face looks more like Maximianus.
Plus the fact that Galerius only had 8 GENIO coin issues 
struck in London as Augustus, all with the usual bust types, 
(laureate and cuirassed, laureate, draped and cuirassed) whereas Maximian was the one who had the spear and shield 
types struck: RIC VI London 8, 18 and 29b.

Sold on ebay in May 2011 for £700.