Sangyo Senshi 産業戦士 Industrial Warrior
"12FG" Why would a Japanese person, a soldier, use an alphabet abbreviation for an English military term? Use of English was prohibited (or severely restricted) during war time, so why would a patriotic Japanese use an abbreviation for Field Gun, especially where there are Japanese terms that would be more acceptable?
What would a commercial or "industrial warrior" be doing with a field gun anyway? If he's manning a field gun, he's not a commercial warrior, he is a regular soldier.
My feeling; the dealer doesn't know what he has, and neither do we. He is telling a story with some plausible bits in order to present his item as a war relic. People who are eager to believe will inflate the plausibility, and their enthusiasm will overcome the incredulity, suspicion and hesitation they should have when buying antiques.