well, I thought that a 16800 is similar to a 16610, and the only difference is the type of steel used for the case. So you need to start looking for a 16610 case since a 16800 case is difficult, if not impossible to find. Correct me if I'm wrong, but a 5513 case is 39.5mm whereas a 16610 case is 40mm in diameter. So you might have a problem of fitting it a crystal etc etc, if you use a 5513 case as a base. Might as well buy a TW best case from BK, drill the holes, change the insert. However, I would ask BK if gen movement would fit or if not you can send it to JMB to mill it. hope that helps.