This example seems to suggest it's specifically a sow's stomach:
“Currently Google's early prototype can only translate German to English; so where it will save the unsuspecting tourist in Bavaria from a slice of gefullter Saumagen (stuffed sow's stomach) the green-faced vegetarian in Paris will be left prodding his plate of pied de cochon (pig's trotters).”
--Telegraph.co.uk: news, business, sport, the Daily Telegraph newspaper, Sunday Telegraph
A fine German dish; sort of a pig haggis. From the examples: "Saumagen is the stomach of a pig filled with chopped up bits of the pig that won't go into any decent meal, plus some spices and onions." What if one doesn't care for spices and onions?