There can be some confusion about where to mount partial plates on a CDI vacuum drum.
Generally there are two main types of vacuum drum. The main vacuum channels can be around the circumference of the drum or along the axis of the drum.
If the vacuum channels are around the circumference, the vacuum holes are normally distributed evenly across the surface of the drum. This means the vacuum is about the same all over the drum, so it is not so important where the plate is mounted. In this case, the plate would normally be mounted at the left side since the carriage starts at the left side.
If the vacuum channels are along the axis of the drum, the vacuum holes can be at left or right side, depending on where the vacuum coupling is mounted. In this case, it is ESSENTIAL that the partial plate is mounted to cover these vacuum holes. So, it can be either left or right side (wherever the vacuum holes are located) and TAPED as appropriate. If this is not done, the plate is not held securely and it is very likely the plate will fly off when the drum spins up. Even if it does not come off the drum, it can lift partly, so the engraving can be out of focus leading to quality problems.