Recently arrived a small, elongated package.
The leather is a bit worn and the embossing faded, but the marking 4088-3S can be read on the flap, with 'K&E polyphase duplex slide rule' on the end.
And inside this 4088-3S leather sleeve is indeed a Keuffel & Esser Polyphase Duplex slide rule, model number 4088 version 3. Quite common in North America, but much less so in continental Europe.
Had not seen or used a duplex before, so took a chance when spotting this basic model. Duplex meaning the slide is usable at both sides, the stock being held together by the metal clamps at the ends.
It was somewhat dirty and 'stuck solid', but cleaned up very nicely. Everything can be screwed apart and cleaned carefully. A basic polish and removing of dirt, using an eraser / rubber to remove stains and even out / lighten the yellowed celluloid. Carefully clean the glass to not accidentally remove the hairline. Putting it all back together again with some care to get the alignments right (well, good enough). On one edge the celluloid has lifted and warped, so re-mounted the cursor 'flipped over' to run on the smoothest side. After the cleaning and some adjustment it slides very smoothly.
The 4088 is a fairly basic duplex rule, many later duplex rules go rather overboard with log-log scales. The front of this rule has, to continental eyes, odd scales; no AB, but folded CD scales (by pi) with an inverted CF. This deviation from the Mannheim is actually very neat and handy for the basic operations - clever.
The rule being duplex, the AB scales have been moved to the back of the rule, with an inverted C and regular D. The reverse also has the K, L and the sine and tangent scales, making it look quite crowded.
The serial number 378790 puts this as an early 'thirties rule - the K&E serials are a bit of an approximation, reading the graphs would make it around 1931-ish. The cursor however has the flanges at the corners to protect the glass from chipping. From the online sources on slide rules (yes, there is definitely a slide-rule-O-sphere on the internet), this type of cursor was made between '33 and '35. Assuming this is not a replacement runner, this rule was likely manufactured in 1933.
After 85 years, still giving results to three digits :-)