What you have is a design or fabrication piece mark. This would typically designate a W16x26, a standard shape available then and now. You should be able to find the properties from a variety of sources.
16B26 was the designation of a light beam in use around the 70's. His section properties are very similar to the W16X26.
An old Bethlehem booklet show the following properties:
Area = 7.65 in^2
Depth = 15.65"
Flange width = 5.500"
Flange ave. thickness = 0.345"
web thickness = 0.250"
d/Af = 8.25
Ix = 298.1 in^4
Sx = 38.1 in^3
rx = 6.24"
Iy = 8.71 in^4
Sy = 3.17 in^3
ry = 1.07"
This section is non-compact for ASTM A242, A440 and A441 steels.
Note the difference in the properties about the Y-axis between this section and the W16X26. I think it is due to the flange of the 16B26 being tapered.
