The only differences at all that are listed in my Sylvania Electronic Products Technical Manual are as follows:
The 21EP4 has what is referred to as a "Cylindrical Faceplate" and the 21ZP4 has a "Spherical Faceplate"
This doesent mean one is a "roundie" fellas because they are both listed and pictured as "Rectangular Glass Type" tubes
Maybe someone out there knows what the faceplate thing is all about because that is the only difference except for a difference in minimum usefull screen dimension of around .25 inch.
Except for the faceplate the tubes look identical but are not listed as subs.
