WWII Japanese artillery is relatively rare.

 

There were never large quantities manufactured and some were outdated designs and calibers exclusive to Japanese forces. Note the prevalence of wood wheels on most. Nevertheless, the artillery was generally adequate until the United States was drawn into the war. 

 

After the war, little use was made by other countries with captured Japanese artillery. With the exception of China and Southeast Asia, the artillery was mostly located on islands which made it difficult to recover.

Interestingly, we have a US Field Manual from the early 1960's Vietnam era which show some Japanese artillery that could have been encountered in use by the Viet Cong.

 

The Japanese artillery in the museum was brought to the United States on empty merchant marine ships.  We know of three that were used as movie props prior to being acquired by the museum. 

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The majority of the Japanese artillery on display was significantly deteriorated from being on tropical islands. They all required an extensive professional restoration which took over a year to complete.