The number of rigs drilling for oil and natural gas in the United States has fallen to the lowest level since at least 1940, oil services company Baker Hughes Inc said on Friday, as energy firms continued to slash activity amid the deepest energy price rout in a generation. Combined rigs in the U.S. oil and gas fields fell by nine this week to 480, overwriting a previous record low of 488 in April 1999, according to the closely-followed industry data.