"Captain Hook" is a solid, single-piece beauty of a tamper. The arc of the hook fits neatly in your palm, so that the extension to the tamper base will fit right between your fingers. This allows you to center your forearm directly over the tamper and push straight down, while twisting the wrist to give you a perfect tamp. This tamper will work just fine in a 58 mm portafilter. (Note that 0.5 mm. = 1/50 of an inch!)
You should read our article on "How to Use a Coffee Tamper Correctly for Uniform Extractions" if you're not exactly sure how to use coffee tampers.