The largest area of the site brings in more questions! I've put these together for one answer as they are all related.
It is not my intention to cause confusion (q3), quite the opposite, in fact! I list all the relevant names and numbers I can find for a battery to make things easier for people.
The fact that no current manufacturers use a particular name/number any more (qs1,2&3) doesn't mean that there is no longer any device that carries that name/number. Many people come across, buy, collect old pieces of battery powered electronic equipment and, often, the only reference to the correct battery for the equipment is the number supplied by the original manufacturer. It is comforting for such an owner, purchaser or collector who is guessing what battery will fit, to find it listed alongside the currently used designation and, therefore, get the correct battery.
What about obsolete batteries (q4)? Well you will find that if a battery is now obsolete I usually have a tip or trick listed to use a battery of today, with a little modification, to fit where the obsolete battery used to (although sometimes this is, unfortunately, impossible).
For some examples see: