This was determined by Kansai Natto Kogyo Kyodokumiai (Kansai Natto Processors Cooperative Association) in 1981, which made a sound connection (nat (= 7) to (= 10)) in order to promote the consumption of natto in the Kansai region.
これは1981年、関西での納豆消費拡大のため、関西納豆工業協同組合がなっ(7)とう(10)の語呂合わせで制定したものである。