This was Ginza Street in Mudanjiang, Manchuria, China. It was under Japanese occupation since 1931, hence the name "Ginza" after a district of Tokyo, and the city was called "Botankou" by the Japanese. That's the reading of the Chinese characters in Japanese.

The top of the postcard gives the location read right to left: Botankou, Ginza Avenue. (牡丹江 銀座 通り) (I've written it here left to right in the modern way)

The shop signs are in Japanese, and your great-grandfather mentions that a lot of the buildings aren't there anymore, so I guess this was a postcard left from its heyday as a shopping street. Here's a 1942 photograph of the same street from Wikipedia.

ETA: I'm rather interested what your great-grandfather was doing there in September, since Mudanjiang a) is inland b) was liberated by the Soviet Union in August.

ETA 2: This may help you with further research. the battle that took place in August is written as "Mutanchiang" in most sources, according to older transliteration rules. The battle was very destructive, no wonder a lot of the buildings were gone when your great-grandfather wrote your grandmother.