That first link has some interesting information / speculation.
The first half of the top line 世界の冠たる can be taken as a translation of the German phrase "Über alles in der Welt", which has Nazi connotations. The company was founded in 1918, which invites the possibility that the logo was adopted during WWII.
All of that indicates that it is clearly more than "Nazi chic" like you would see hawked to edgy teenagers. On the other hand I wouldn't assume that the one-time owner was a Jew-hating ideologue; more likely he admired German culture, adopted the logo because that represented Germany at the time, and saw no reason to change it even after the war.
The second half of the first line says "Kawanishi's products". The text below the logo is just the company name, what they are, address, and phone. The address includes a district that was abolished in 1989, so the sign was made before then.