You have confirmed it is not a fake. They are rare by any measurement. How many 78 year old, 28 gauge guns do you think exist. The 28 was not that popular at that time. Call it a elite gauge or a snob gauge. Skeet was a very minor game in those days and most shooters just used a 3" .410 gun in the 28 ga event. So there were not that many made in the first place which is why so many 16 and 20 guns are "converted" to pretend to being a factory 28.

I have a factory 28, much later production, that I would not take the BB for. Rule number one for me when considering value of a gun either selling or buying is how likely am I to ever replace it. So if you like it and can afford it buy it because you might never find another legit one like it.