I'm pretty sure the later MK1 ignition barrels were the same as the MK2 ignition barrels. They both were larger diameters and were located on the steering column. The early MK1 ignition barrels were smaller diameter and were mounted on the steering column plastic.
The MK1 and MK2 steering columns are also both interchangeable but where the steering wheel bolts onto is a smaller diameter on the MK1.