I hate to ask, but is this a reference to the comedian Brian Regan's story about grade school?
In the story he states that he was asked what the plural of 'ox' was. He didn't know, but this kid, Erwin, who knew everything informed the teacher that the plural of 'ox' is 'oxen'. She then turns back to Brian and asks the plural of 'box', to which, he replies, "Boxen... I bought two boxen of doghnuts"
I always think of that routine when I hear boxen...