Oh wow, sorry OP. This is getting out of hand. Keep in mind that even if 501 cartos fit the battery, it's likely to have the opposite polarity of a 510, so you can't use the charger for 510
batteries even if it fits. There is much uncertainty as to what you really have, so I wish you luck in figuring it out.
If it helps, I found this picture. It seems 808A and 808B are the same, just different battery lengths.