1. At first sight this rule looks like a fairly standard Reitz pattern plastic rule - but as sometimes happens with slide rules what is seen at frst sight is not everything.

2. The first clue is the stamping on the back . The upward pointing arrow indicates that this rule was at some stage was the property of Her (or possibly His) Majesty's Government. If the "53" is the year that would consistent with other indications of the date.

3. Another interesting feature of the rule is that on the back of the slide there are three scales: for Sin, Tan and small angles for Sin and Tan. A normal Reitz rule would have Log as third scale. In this case another unusual aspect is that whilst a slot is provide to see the back of the slide, there are no marks on the cut out portion to enable the slide to be used without removing it and turning it over. It would appear therefore that the rule was probably intended for artillery use.

4. The back of the rule also carries the patent number 644944. This patent was taken out in 1950 by Blundell. It is for a method of adjusting the tightness with which the slide in held in the stock. A diagram taken from the patent is shown below. As can be seen it takes the form of a very asymmetrical "L" which can be turned to squeeze or release the slide.