I believe they were introduced about 1962. Originally plain polished alloy later ones had a coating of some sort applied. The most modern ones have NRS (National Rail Supplies) on the side instead of BR. Some were also supplied in a yellow coating. Many were fitted with an extended knob to prevent them being turned onto the yellow aspect ,also a washer with a lug was also fitted internally to prevent a yellow being shown. The army issued lamps were painted green with the army arrow on the side.