I have recently added to my small collection of KLG literature and amongst them is an October 1934 dated booklet detailing the 5/- KLG - Effectively the L777. KLG had for years promoted their 2-piece 6/- K series and 7/6d KS series plugs and the high prices were costing them sales so they intoduced the three-piece L777s for sale at the 5/- price level of the competition. It seems that this plug which they claimed had a more restricted heat range was suitable for most standard engines. As I understand it, this was the recommended plug for KM and WM20s up until at least 1942.