I have a rear hub from before 1932/3 (think it's a two speed) that I'm hoping someone will be able to date. This one has just the six figure code on it: 683820. There is nothing else at all on the hub aside from these six digits. If it's an SA, it pre-dates the K-series code but I don't know if it's simply a production number or whether there is some other date code at work. Any help would be much appreciated.