I worked for Plessey, when barcodes were first introduced in Sainsbury's for stock ordering. I used to visit branches across southern England to maintain the datacapture trolley's. Product barcodes were located on the shelving below the items. Staff would do a daily stock check, scanning the barcode for items that needed replenishing. The information was recorded on a cassette tape in the trolley, and was transmitted in the evening via a 1200Bd transmitter to Blackfriars. Later the barcodes were also used for shelf checking to ensure the price displayed on the shelf corresponded with that being charged at checkout.
Image contributed by Peter Coleman