FWIW, i read the name on the death cert as Carl Schwertfeger, also.
In the German telephone book (dastelefonbuch.com), found 0 LEHWERGIGER; did find 500+ SCHWERTFEGER.
Also found 2 locations for Sievershausen: 1) 37586 Dassel, in the Reinhardswald, near Solling; 2) 31275 Lehrte, on the A2 between Hannover & Braunschweig. Both are in Lower Saxony (Niedersachsen).
In a small village, there might be more than 1 family with the same surname, so it would not be uncommon that bride & groom have same surname. Have found that fairly frequently with my wife's Scottish relatives up in Nova Scotia.
Have also found that some family legends are actually myths.
Kenneth Thompson
Moline Acres (north StLouis County) MO