Miniature portrait of Victoria 12 for a 17? (10)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Miniature portrait of Victoria , 12 for a 17? The reference is to the penny black, the first UK adhesive postage stamp, which bore a miniature portrait of Queen Victoria; in old money a bob (=entry at 17) or shilling was worth 12 pence, hence 12 penny blacks could be bought for a bob!