The 8XP4 is the rectangular version. Max 22 kV:
http://tubedata.tigahost.com/tubedat...168/8/8XP4.pdf
5AXP4 is round. Max 18kV:
http://tubedata.tigahost.com/tubedat...68/5/5AXP4.pdf
The rectangular 8XP4 will work in round CRT sets, but the deflection angle is different so the image will be clipped or distorted.