Guinea (British coin)
Although guinea coins had not been made for a long time, prices often continued to be shown in guineas until British prices became decimal in 1971. A guinea was worth £1,1s0d (one pound and one shilling). This is the same as £1.05p in modern money.
Because a guinea was close to a pound, putting prices in guineas made the price seem less. A price of 39 guineas might look like "about £39". In fact it is nearly £41.