B-17 Stenciled marking question

[SHAEF1944]

Hello folks. A couple of us have been doing research finding pics of planes for people who's relatives flew during WWII. While going through a lot of nose-art pics, we have run across some markings we cannot identify.
On the left side nose of a VERY FEW B-17G's, just aft and somewhat on the level of the pitot tube is stenciled " P38 " or "P39", with another number underneath that, ranging in the pics we have from 9 to 290. Looks like this :

P38
9

These numbers appear to be about 5 to 6 inches tall. On most of the A/C there is also a 3-number tail number forward of this stencil, usually twice as big. On one plane, this " P39 " marking is also visible on the tail, under a Bomb Group Square and above the S/N.
We have also run across pics of 2 B-17F's that have the nose stencil " P22" with no number underneath it. The F models appear to be Boeing built, while all the G models we can trace with the marking appear to be Lockheed-Vega built, and seem to have been ferried to the Lockheed Modification Center in Dallas before being assigned. I know the Mod. Center in Dallas did radar-bombing and photo-recon mods on P-38 Lightnings, so my 1st guess was it had something to do with " Pathfinder ", but we've determined that at least one A/C with the markings definately didnt have any BTO equipment. Also have about ruled out Project Number, as these look like they always were stenciled above the A/C data stencil block.

Any of you guys have an idea of what these numbers signify ? Factory or Mod. Center markings, or some kind of tactical data ? We've tried several BG Assn. websites, Boeing, Lockheed, etc, but so far, no luck. Any info would be greatly appreciated, we are flat stumped on this. Thanks