1. This another case where I show three rules made at different times - in this case separated by over 20 years. The differences are relatively minor; different labelling of the scales, a different arrangement of the window for the log-log scales and additional hair lines on the cursor.
2. This rule is a Darmstadt - named after the technical college in that German city. Interestingly the city of Mannheim is not far away and whilst this is also the name of one of the other standard slide rule scale layouts there is no (direct) connection; Mannheim was a French artillery officer.
3.  The log-log scales are on the rear of the slide and accessed via a plastic "window" on the back of the rule. This makes good sense, as whilst the log-log scales  are used in conjunction with the C and D scales for raising numbers to a   non-integer power, the result cannot be carried directly forward into other calculation. For example to calculate 2.5 * (6.1)^0.75 you would have to calculate the value of (6.1)^0.75 first, note down the answer and then multiply that by 2.5. Perhaps this explains why this layout was so successful over so many years.
4. The log scale and the trig scales are on the top and bottom edges of the ruler respectively and are accessed by extensions to the cursor. Although the cursor construction changed with different models, the same principle continued to be used.
5. The cursor construction in the 1945 model is perhaps the most unusual as it used metal clips to hold together the three pieces of glass.
6. The labelling of the scales also evolved. The makers name does not appear of the face of the 1945 model. It also seems strange that the D and P scales should be the only scales with labels on the earlier versions.
7. The stock and the slide are of wood and have metal inserts. The later model has added plastic at the back of the rule. The 1945 model I have, in excellent condition overall, is slightly bowed as if the plastic has shrunk more than the wood. It could be that the extra plastic was added to maintain a better balance and prevent this. Though that said the 1952 model has the same construction as the 1945 model and does not show any bowing.