http://www.simpsonltd.com/product_info.php?cPath=156_170&products_id=49994

"7.63mm Mauser; 80% blue, good bore, fair stock, 11 13/16'' barrel, Rare transistional production with features not found on standard production examples, a longer chamber area on the receiver, and slightly heavier rails without lightening grooves. Large ring hammer with early variation safety, trigger, extractor and firing pin. This carbine is pictured and described in the book "System Mauser" by John W Breathed Jr and Joseph J Schroeder Jr on page 166. Original blue finish, walnut stock and forend. British retailer markings on left side of frame "WESTLEY RICHARDS & CO. / LONDON". British proofing and "Not English Make" markings on right side of receiver and frame. Top of chamber marked "WAFFENFABRIK/MAUSER/OBERNDORF a/n". Matching serial numbers including the stock, serail number "41". Hooded fixed front sight with fine post and bead. Tangent rear sight also drift adjustable for windage graduated 50-500 meters. The elevation adjustment is marked "WESTLEY RICHARDS &/Co". The metal finish has scattered surface spotting, and some corrosion/minor pitting on right side of frame. Handling scratches and dings. The forend has a split on the left side over the receiver rail and an old sliver of wood missing on the right side at the joint with receiver. The stock has the original checkered hard rubber buttplate. Mechanically fine. The bore has strong rifling, showing frost and some minor corrosion. C&R/FFL, s/n 41 "

and here is SN 042 http://egun.de/market/item.php?id=6211895