There are many different variations on Cortina MK1 steering boxes. Here is a brief overview of how to identify what you have.
Actual steering box.
I know three variations of the actual steering box itself. The main difference is the GT and non-GT boxes. A GT box should have a large pin sticking out of it. This acts as a pivot for the handbrake linkage for a dash-mounted handbrake. That's the first thing I'd look for. Other differences occur in the actual shape of the box. There are slight variations, probably between different castings or different manufacturers. The most common seems to be Burman.
The above photo shows two "early" steering boxes and a late one. The top one is "early" and is a standard box. The middle one shows a slightly different casting (there is a "square" sort of box shape near the bottom of the steering box. The lower steering box is "late", and is a G.T. box. Note the pin sticking out for the handbrake linkage. Notice the top steering column. It has two sets of brackets which hold the indicator assembly. The middle one has the brackets missing (some-one removed them), while the later, lower column has one bracket nearer the top. This mounts the later indicator/light assembly.
There are often year dates stamped on the steering box, (for example "65" indicating 1965) but these can be hard to find due to dirt, oil etc.
Steering column length.
There are at least two different length steering columns. The longer one is some two inches longer than a "shorter" one. Sorry this is a bit vague, but I am still trying to find information. The long measures just under 42-1/2 inches, the short is just over 40 inches