This is the lining of the lamb's stomach. It is similar to beef tripe but there's only one kind and it's a lot thinner. It does not, however, take any less time to cook, in fact it seems to take more. The photo specimen was about 16 inches by 14 inches and weighed 1-1/2 pounds.
More on Lamb Innnards.
Lamb tripe straight from the lamb is dark olive green, crusty and needs a lot of scraping and cleaning, just like tripe from a cow. The lamb tripe sold in California is already factory cleaned and pretty much ready to go, but is generally not bleached as white as cow tripe is.
Incidentally, natural unscraped green tripe from lambs and cows is a common ingredient in upscale dog foods, and that's the way the dogs like it, but people prefer it well scraped and cleaned with all the green gone.
Buying: Lamb Tripe is to be found in markets serving a Middle Eastern, Persian, Pakastani, Armenian, Turkish, North African or other lamb focused community. It will generally be packaged one piece to a foam tray.
Cooking: For best results, follow this method: