As you can see (above if your eyesight's good, or below with less eyestrain), in the original tap symbol (as seen on the Revised Royal Assassin), it's a slightly tilted capital "T" -- standing for, of course, the word "tap." Starting with Fourth Edition in 1995, the tap symbol changed to the curved arrow symbol, with a black rectangle representing the tapping card behind it. This change to a more symbolic tap symbol made Magic cards more translation-friendly. Then in 2003's Eighth Edition set, it changed to the simple, straightforward, easy-to-see curved arrow, without the rectangle behind it. See below for high-resolution versions of the three designs.
The Changing Tap Symbol
Posted in Arcana on July 12, 2004