It is a process lens, won't cover 16x20 at infinity, let alone 20x24. Tessars aren't particularly symmetrical but it seems that Apo-Tessars -- mine are all Taylor Hobson tessar type process lenses, not real Zeiss -- hold their corrections well at infinity.

If yours is like the few CZJ Tessars I have, the cells should screw out of the barrel, the singlets in the front cell should screw out and so should the rear cell's cemented doublet.