This article has info about the 8XP4 testing tube and what it's good for:
http://antiqueradio.org/8XP4PictureTube.htm
This article about the 5AXP4 shows a typical kit with carrying box and cables:
http://antiqueradio.org/5AXP4PictureTube.htm
Handy things to have if you do a lot of work on vintage TVs.
Don't pay too much. They pop on eBay regularly and may sell as low as $20 or $30, although your mileage may vary.
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html