Allectus. Double quinarius, London mint, AD 293-296. 5.74 g. 

IMP C ALLECTVS PF AVG, Radiate and cuirassed bust right.
VIRTVS AVG, galley sailing left, seven oarsmen, double waves below. 

Mintmark QL. 

RIC V 55 var (denomination weight); Burnett, Allectus 111 var. 
Apparently unpublished double-weight specimen.

The VIRTVS AVG galley type is unknown on his antoniniani. The average quinarius weighs about 2.56-2.76 g and most antoniniani  4-5 g. The mintmark tells us that this is a quinarus, yet this coin is about twice the weight of the usual Allectus quinarius. 
It may be an unpublished denomination of Allectus, the double quinarius.

With permission of CNG (Mail Bid?) auction 70, lot 1061, Sept. 2005.
Sold For $650