A person who does not know a company called 551 Horai in Osaka is not a citizen of Osaka. Probably all citizens of Osaka between age of 20 and 60 know this company. Then, if you talk about butaman(steamed pork bun) in Osaka, you are talking about the product of this company.
551 Horai, the Japanese most famous manufacture of butaman. The steamed buns filled with pork are normally called as NIKUMAN(meat bun) in Tokyo and sold as nikuman in convenience stores throughout Japan. Niku means meat in Japanese but niku is mostly recognized as beef in Kansai area including Osaka. Therefore, nikuman is considered as steamed bun sold at convenience stores or in supermarkets but manufactured by non-Osaka based major food companies.
3 Chinese immigrants from Taiwan opened a Chinese diner called Horai in Osaka in 1945. They expanded the business and operated several diners in Osaka but eventually broke up in 1963 and established their own individual company. One of the founder of Horai, made the trademark as 551 Horai and started to sell pork buns for take away.
551 Horai operates more than 50 stores selling pork buns and other dim sum in Osaka and its surrounding. It is possible to buy chilled pork buns(not ready to eat) at 8 stores including at Shin-Osaka station.