1. These represent two variations of a rule described as "Rietz"
2. The scale configuration is identical on both rules.
3. Some of the differences are:

• Use of colour (red/black) on the 23R
• Use of scale extensions on the 23R
• The cursor of the 23R has three full length lines: the 0239 has one full length line and three short ones. In the case of  the 23R they represent p/4 or Öp/4 depending on whether the A/B or C/D scales are used. In the case of  the 0239 two of the represent the same values but for different scales and the third is for HP/kW conversion.
• The tables of constants on the back are different - though this may in part be due to the fact that the 23R was produced for the German market and the 0239 for the English market.

4. The front of 23R has hand scratched reminders of rules for decimal point.