[BK0scillation]

Flag batteries may well have had other uses,but one of the main was for signalling, in country houses, hotels and the like you had an indicator panel with a flag or some kind of mechanical indicator and a bell which summoned servants/hotel staff, for those that have not seen them some thing like 9 inches high and about 3 inches diameter,with two screw terminals on the top generally sitting on a shelf, using a number in series, according to use they would last typically ten years plus, this would be in the days prior to electricity supply
I found some new (old stock) about 1966 when a youngster, they even used grid bias batteries I suppose when the war? made these hard to get,
Jeff G0PGY