An excerpt from a letter to the editor in a recent issue of The American Philatelist:
"Before decimalization, the single word for multiple d coins was pennies, not pence. Pence was only used in combination - sixpence, thruppence, and then usually only to designate the coin. If you had a sixpence, you could buy six pennies' worth of sweets. A stamp selling for 5d was called a five-penny stamp. Pence as a standalone word only came into common use following decimalization. If you asked for a 5 pence stamp before, it would have sounded odd. Now it is the norm. I think even now, pence should be a plural noun with penny being the singular form. "A pence" sounds very strange to me. And any use of pence as an adjective seems odd, such as "shilling and pence stamps."