I have three different generations of Rex Ambassadors - here are their differences.

The one on the left is from the first batch and it's a M3, the middle one is from a year later and it's a N3 and the right hand one is a R4.
(Each image below has them in this order.)

I'll start with various measurements

Moving onto the visual differences.

Starting at the top.
The topcap on the M3 is thinner than the the other two top caps. The other two measure the same and could interchange.

The head itself gets thicker each time. The M3 is the thinnest and the R4 the thickest. As the topcaps are the same the difference comes in the middle plate bottom plate combo and as they don't come apart, I can't say where the difference is.
The little section between the head and the adjuster dial is biggest on the M3 and the later two are shorter and as close to each other as not to matter.
When we get to the knurling on the adjuster dial, they are all different. The early one is the coarsest, the N3 the finest and the R4 is in between the two.

Moving slightly back up and looking at the bottom plate. The M3 has squared off lather holes (ah-la Gibbs) while the other two have rounded ones.

The picture above shows the difference in the topcaps and also the bottom plates. The M3 has ridges on the edges of the bottom plate and the other two are smooth edged.
The N3 and R4 topcaps have a recess in them while the M3 has none.

A side-on view shows the differences in the bottom plates. The M3 is the thinnest, while the other two measure within 0.05mm of each other.
The length in the sections under the head can be easily seen here as well as the coarseness of the adjuster knobs.
The lengths of the handles are 82mm vs 79.5mm vs 79.6mm respectively. The knurling on the handles are also all different with the pattern following the same pattern as the adjuster knobs. M3 been the coarsest, N3 the finest and the R4 in-between the two.
The knirling on the twist knobs of the N3 and R4 look very much the same while the M3 has the coarsest knurling.

This image pretty much shows the same details as the previous image, but from a different angle.

Once again the different lather holes are very clear here.
However there is a big difference in the finish of the side tabs of the middle plate. The M3 is similar to the Gibbs while the other two are more "beefy" and have internal edges that are rounded off.