WC is the nominal outlet pressure
You can get convertres with different secondary springs.

Standard Cobra=
Blue spring: -0.37 kPA (-1.5 w.c.)

Cobra I=
Orange spring: -0.12 kPa (-0.5 w.c.)

CobraII=
Red spring: -0.05 kPa (-0.2 w.c.)

Model Js have the same optional springs and pressure ratings, I usually just use Cobras. There are only slight variations in the dimensions. 70.3mm vs 73mm overall depth, 117 mm vs. 118mm overall width. These ratings and dimensions are from an IMPCO Catalog that I just happened to have sitting in front of me at my home computer.

If you are working on some carb systems there will be an additional vacuum port on the front cover of the Cobra Converter. The computer determines the mixture requirements from the O2 sensor, and in some cases it combines a map sensor, to energize the fuel control solenoid valve. When energized it puts vacuum on the other side of the diaphram, hence the vacuum port, which essentially lowers fuel pressure and leans out the mixture.