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I bought one of the Lansky World Legal knives as here;
https://www.amazon.co.uk/Lansky-Sharpeners-LKN333-World-Legal/dp/B00IJ93WW2/ref=sr_1_1?ie=UTF8&qid=1532382291&sr=8-1&keywords=lansky+world+legalThe blade is only 2.75" long but still quite hefty in width and all though it does not lock the slip joint is very stiff, I don't think the Old Bill would look to kindly on it if you were stopped, I keep it in my tool box at work :)
Spyderco UK pen knife is pretty awesome.