I have the same Concise No 28 as James Taylor describes. The back has 38 different unit conversions in pie wedges. I always thought that was cool enough. But we just discovered that most of the conversion wedges have a code indicating how to set the rule. For example, to convert Miles to Kilometers There are 1.6093 kilometers per mile. The wedge says "87 opp 140". When you set 87 on the inner C scale opposite 140 on the D outer scale, you get 1.6093 kilometers per mile. Once it is set, you can read miles on the C scale and get kilometers on the D scale or vice versa...for any value of miles or kilometers. Same for most of the other conversions. The OPP settings are integer readings on both scales, so the owner does not have to estimate a decimal setting that leads to imprecision. A very helpful feature I never knew about.

I have one from the mid 60's in college. Memory does not place the source but it is a little different. The front matches your picture but the rear has a large number of English/Metric conversions, weight, distance, speed, etc. The small 17 page manual indicated is was made by the Concise Co., Ltd. in Edogawa-ku, Tokyo, japan. It came in a black plastic case marked, Concise (in script) No.28. It along with a 10" stick was used until I could afford a Scientific Instruments Co. no 1610-D, 5 in overall diameter, slide rule. I still carry both with me to show some of my college students what we used back before the internet came alive.