Propertied Interest

Solution 1:

Used in this way the propertied interest means the interests of the people who own property. For this purpose it would mean landed property, i.e. ownership of land and buildings.

Propertied interest is a well-understood, and everyday idiom, particularly among historians, social and political scientists etc.

Some, especially those subscribing to a Marxist theory of history and society, talk of a propertied class.

Solution 2:

It appears that "interest" is being used in sense of political power block (though not necessarily one directed at government); a "special interest." MW

Accordingly, your sense that it means the "interests of the proprietors" is quite correct.