Historically, Japan got their hands on the Bofors when they captured Singapore, but they only managed to produce prototypes of the Japanese model by 1945(something that is a bit surprising - the Bofors was well-suited for Japanese industrial processes, being a gun that was largely hand-built). This tells me that, to have the Bofors in time to make a bit of a difference, they should get it by 1939 at the latest, by 1937 would be better.
This limits a bit the Japanese options - getting the blueprints from the Polish would be about their last chance of getting it in time for WWII. This is why I mentioned the German 37mm guns - I don't think the Japanese would be able to get their hands on the Russian 37mm in Nomonhan - it would have been a newly-introduced gun by then(and Lake Khasan was earlier than its introduction). OTOH, German 37mm guns, whether the most common Flak 18/36/37 or the rare M36 Navy gun(the Bofors analogue which would be the base for the Flak M42 Navy gun) would be available in time for Japan to establish a production line before WWII.
I don't think the Japanese would go for German guns though, at least not before 1937 - historically, the Japanese arms industry had ties to French and British industry, and they would go for guns of those countries preferentially - the Type 95/96 was a replacement for the 2-pdr Pom-Pom in IJN service. I think Schneider had a decent 37mm gun that they exported in small numbers, but I'm not sure.