[MrFixr55]

Hi Brian, 

It seems that this CRT may be a little rare.  20" units were not as popular as 21" Units.

The following is from the Cisin's Tube Sub Guide from Early Television Foundation.  Theoretically, everything in this group 25 that matches all parameters (including those not listed) is a fit.

The problem is this:

• 21EP4 is not in this group.  
  
• A 21EP4 has a cylindrical faceplate.  The surround (mask) will not fit.  
  
• Electrically, anything with Base Diagram 12N is the same wiring, but electric similarity is not guaranteed. 
  
• The dimensions of the faceplate in a 21EP4 are going to be bigger by 5/8" vertically by 2" horizontally.  This will require modification of the mounting.
  

• Per the Cisin's manual, the 20DP4 has a spherical faceplate. A 21WP4 has the same base diagram and dimensions at the faceplate but the tube is 1/2" longer.  In addition, although it is in the same group, it does not show as being a direct swap.  And the fact that yours is listed as a 20" but the 21WP4 should be a little bigger makes me wonder if it is a misprint.
  

• ETF has a 21ZP4, used. This has the same dimensions as the 21EP4 (bigger by 5/8" vertically by 2" horizontally than the 20DP4) but has the cylindrical faceplate. the Base Diagram of 12D indicates that it does not have an external Aquadag coating, but this can be applied.
  

Do yiy collect TVs?  Do you have another TV with a somewhat similar tube (55-90 deg, magnetic deflection, magnetic focus)?  You can extend the wiring from the Philco and connect 2nd Anode, Electron Gun, Yoke and Focus coil (and Ion trap as needed).  The purpose of doing this is to test the rest of the set so you know, that in addition to the work and expense of replacing the CRT, what else you are up against.

Keep the info coming, and good luck!!