I think this might be it...
• P is the 3 least-significant digits of mean sea-level
pressure in whole dekaPascals. To the left of the 3
digits, prefix either “9” or “10”, depending on which
one gives a value closest to standard sea-level pres-
sure. For kPa, insert a decimal point two places
from the right. For hPa, insert a decimal point one
place from the right.
Example: 041 means 10041 daPa = 1004.1 hPa =
1004.1 mb = 100.41 kPa. New example: 957 means
9957 daPa = 995.7 hPa = 995.7 mb = 99.57 kPa.
[CAUTION: Some organizations report P in inches of mer-
cury (in. Hg.) instead of hPa. P MSL (in. Hg.) is an altimeter
setting, used by aircraft pilots.]