Bottom of this thread discusses 7jp4 vs 7ep4 vs 7gp4
http://videokarma.org/showthread.php?p=3027180
lifted from link:
"come across a Transvision that had a 7JP4 installed in it, worked fair.
7JP4 2nd anode is 5 to 6KV, 7EP4 2nd anode is 2KV.
A 7GP4 is a closer match only requires 3KV, but harder to find than 7JP4. Usually found in Philco 48-700 sets.
Better temp tube is a 5BP4, easier to find than either of the others and is electrically identical to the 7EP4"
================================================== ===
Also 7GP4 was used in Philco 48-700 in early versions and Philco substituted 7JP4 in later versions.