This was my original thought as well. The biggest thing about this is -- if this were the case, that'd mean their original plan with Mirrodin was to introduce Equipment and Vehicles in one set, which seems kind of like a lot (especially if there's some type of creature-attaching mechanic here.)
It also doesn't quite solve the core mystery of Mechanic E, i.e. why it's designated with a letter. In the article where this mechanic was first mentioned, it showed that other letters are typically used in R&D either to indicate a card frame or a symbol (mana, tap, etc.) They weren't as custom-frame-happy in Mirrodin block as they are now (the first real set to introduce any since gold cards was Ravnica with hybrid) so I'm not sure this would have really been a "custom frame" type mechanic in 2003, and they rarely introduce new symbols for things they could spell out, so... questions.