After the cryptic responses faded into the background, a helpful member over on MTBR posted up this page from the WTB catalog back in the day. With the P2 I'm using, I've got about 11 7/8" (11.875") bottom bracket height, and according to this sheet (for 1999 non Phoeni labeled frames, but with the same geometry I understand) My 18" frame should have 11.688 inches. My eyesight is bad enough to explain the 0.187" discrepancy, so...consider the "mystery" solved.
The 410 a-to-c P2 is spot on the a 1997 18" Phoenix. Done and done.
And a shot from the previous owner, The Phoenix in a previous life.