[MrFixr55]

Hi Brian,
"Curved horizontally but flat vertically" describes a CRT with a "cylindrical" viewing surface. If you look at the "mask" (the trim piece that surrounds the face of the CRT)
The DuMont that almost blinded me was meant to have a "cylindrical" Tube. A "spherical" tube will fit the mount but will not line up fully with the mask if the mask was meant for a cylindrical tube, and visa versa.

The schematics that Terry "RadioRoslyn posted for your set lists the CRT as a 21EP4A. According to my RCA Receiving Tube Manual, version RC24, a 21EP4A is a cylindrical face CRT, magnetically focused, with 70 degree deflection angle and with base diagram 12N. However, I kind of agree with the others that your tube looks to be spherical in the picture, but it is not always easy to tell. Per my RCA Receiving Tube Manual, 21WP4 and 21ZP4 are spherical face 70 degree tubes with magnetic deflection and focus, and base diagram 12N.

One other thing to check, since your current tube is a name brand (GE) and not an off brand, is that the tube number may be stenciled on the neck of the tube, similar to the way it is done on a regular tube. Using a flashlight in a darkened room, look for the number on the neck area of the tube. Be careful wiping the area as you may wipe off the numbers.

Good luck.