Listed as a Frankonia gun - that company has sold all manner of firearms and scopes made by other companies with their own branding - I've seen Zastava, Geco and S&B labelled as such...

I've seen a few Mauser action rifles fitted with .22LR barrels underslung in a similar manner but 20g is new to me.

For the price he is asking though you could buy one of these new https://www.innogun.de/en/jagd/rifles/hybrid.html