What is the "temperature code" of a soldering tip?
On the WTCP tips, the "code" is simply the temperature /100 in degrees F where it controls. So a '6' tip would control at 600°F or about 315°C. 

They are available in 600/700/800°F. See the datasheet for the Magnestat irons and tips here.

The way these work is that there is a ferromagnetic part of the tip that is designed to lose its magnetism at the specified temperature (the Curie temperature - named after Pierre Curie) and a magnetic switch lives in the handle to turn the heater on and off.  

The 600°F tip is fine for repair work and small scale production using  63/37 SnPb or 60/40 SnPb solder. For lead-free solders or for fast production work (skill matters more when then tip is hotter since it's easier to damage the board by holding it on too long) you may wish to use a higher temperature tip. 

The tips should not be hard to find nor should they be expensive- there are many, many of these irons in the field, though most new ones probably use electronic temperature control rather than the Magnestat principle, hence different tips.