1. This seems like a straightforward basic slide rule (see list of scales) with nothing remarkable except that it has two cursors and an "inverted" log scale. What makes it more intriguing is that the cursors are not identical.

Features of the cursors which are the same are:

• Dimensions and material (aluminium) of front plate
• Position, size and material (brass) of the four screws on the front plate.
• Spring and method of fixing (river at right hand end).

The differences are:

• Degree of use - one (shown on the right) has much more "gunge" than the other. (I deliberately have not cleaned them).
• Construction - the one on the left is all aluminium the other has painted wood for the top and bottom runners. This can be seen in the detail of the back view of the cursors.

I do not know whether I have got a rule intended for some abstruse function or whether I have got myself a spare cursor!

2. The log scale, on the back of the slide, runs from 10 to 0 (see detail below). Is this a clue to its use? On reflection, probably not as I have other early PICs with the same feature.

3. The rule has the letter P blind stamped on the back. I have not seen it on any of my other early Thornton/PIC rules though in some cases it may be there but hidden by the list of conversion factors, which judging by the rectangular stains on the back was originally present on this rule. By coincidence a few days after buying this rule I bought a much later Thornton AD 050 (circa 1970) which has an identical "P" stamped on it. Again it is the only one of my later Thorntons with this mark. The two rules were probably made about 40 years apart. I have no idea what, if anything, the "P" means.