This picture shows the second processors with their lids off. The date codes on the chips indicate that these machines were made in 1984 although the one on the left is slightly earlier.

Can you spot the difference between the two?

If you look at the back of the units just above the ventilation slots, you can see that the one on the left has two cut-out holes in the casing but the right-hand unit has three holes.

These are unused on the Second Processor but I think Acorn must have used a standard cheese wedge housing for all the units like this, including the Teletext Adapter, Prestel Adapter, Z80 Second Processor and so on. Evidently the casing must have been revised at some point in 1984.