They made two different types of the 511, curved back like yours & straight back & there may have been mold base variations as they are early. Also they must have had a limited production run because we don't see them much in the U.S. however not that unusual as its well documented that some Cambridge designs that didn't sell favorably here in the U.S. sold very well in the London showroom over the years. Ones like yours are difficult, to say the least, to find. Later on Cambridge went with the Pouter Pigeon, Scotties, Eagle & Lion bookends...figural bookends with better sales results. The London showroom was especially successful with the Cambridge figural flower frogs.