It is I belive an older line not seen too much in 1 or 2 generations back of the pedigree's today. Joseph Heine is the breeder who serves as an American Kennel judge and raised the Boxella Boxer line in the US, a line originating in Germany.
A search for "Boxella Joe Heine" will bring up some information. There was a profile/tribute in The Boxer Ring magazine earlier this year (www.theboxerring.com). Some Boxella dogs are behind some recent top Boxers, and unfortunately "Boxella" has become a popular buzz-word with some questionable breeders who like to kennel-name-drop.