Hello, MIke,
P53S 026901 - you almost got it right
P 53 S 02691
P = 1.5 ton 4400
53 = 1953
S = St Louis, MO assembly plant
026901 = 26901-1000 = 25901th truck (all sizes) out of that plant in 1953
In 1953, the serial number sequence was for all trucks assembled.