When I was a child we had milk cows . My mother would churn the cream off the milk to make butter . She would add salt to the butter ( in a large bowl ) and work the butter to get all the buttermilk out of the butter . I think what you really have is a butter mould to shape the butter into a block instead of just having a lump of butter . By the way homemade butter is very good if done right . Mother also made cottage cheese from the cows milk . Store bought cottage cheese is no comparison .
I changed my mind about yours being homemade because the corners appear to be dovetailed .
Best Paul