21/-?Prior to decimilisation (1968-1971), UK books were priced
in shillings, or shillings and pence, where 20 shillings
equals one pound and 12 old pence equals one shilling.
Shillings were indicated with a variety of suffixes, e.g.
3s, 3', 3", 3/ all mean 3 shillings. Any number after that
is additional pence, usually 6 (half a shilling) but
sometimes 3 or 9 (a quarter of a shilling or three-quarters
of a shilling).