The Lamb Meat Buyers Guide NAMP (National Association of Meat Purveyors) and the IMPS (Institutional Meat Purchase Specifications) use identical specs, but the IMPS is put out by a government agency and is free. Click on the USDA IMPS for Fresh Lamb/Mutton link to download the 2014 PDF version of the Lamb file which includes NAMP/IMPS item numbers and descriptions of the cuts. It will open in a new window, but it is a 1 MB file so it may take a minute to download.
Lamb Offal Specs
For Lamb Offal click on USDA IMPS for Variety Meats which also includes offal for Beef, Pork, Veal, Cured, Smoked, Sausages and other by-products.