Part Two: Differences between the Regular, Classic & 21 model Sebenza:

So start off, here's a picture of all three together - The Regular is pictured top, the Classic is shown middle and the 21 is bottom.

By the way, designs can vary greatly within similar models. For instance, any of these knives can be purchased (some second-hand only now) without handle inlays so will have plain titanium scales or will maybe have etched graphics there instead. Blades will also vary in steel and pattern but will all retain the family shape shown below and you can have single or double thumb lugs on any blade in colours such as blue silver and gold, like the lanyard attachment and backspacer which are also available in differing colours. I guess what I mean is look at the overall shape to identify the model.
The Classic & 21 blades are the same and there is a noticeable difference between these and the Regular. The Regular has a step on the spine and has much most belly to the cutting edge. The Classic looks slicker to me with it's more even spine drop and it appears pointier too, as a result of that feature.

Between the older Classic and the newer 21 model - on the scales, the difference there is that the 21 has a deeper pivot end bevel (both front and rear) which was designed to make it more pocket friendly and it also did away with the rear scale writing. MM is there as a Roman numeral acknowledgement for the year the Classic model launched.

There are other things to note. One thing which did surprise me was that the Regular Sebenza is slightly shorter. (Top - Bottom = Regular, 21, Classic)

As you can see from these two pictures, the classic and 21 blades follow each other perfectly while the Regular with it's bulkier looking step drop spine is different. (Top - Bottom = Regular, 21, Classic)

A final picture to show the deeper pivot end bevel on the 21 (middle) On a side note you will see the edge ridges which the regular Sebbie used to have, now discontinued, and the different thumb grip cut-outs which were tried out on a limited run of all Sebbies around end 2004 into 2005. Both features are shown on the Regular (top)