Hmm, good point. I ran into that issue once when using a rebuilt tube that had the right electronic characteristics but a slightly different bulb profile than the original.
Most of the dimensions look close enough, but now I see that the diameter at the 12KP4A screen face is 7/16" wider than the 12JP4. Of course, the dimensions given in the data sheets are plus-or-minus some fraction of an inch. I'll investigate whether there's enough wiggle room to accommodate it.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html