Normally the creator of forged documents doesn't want the true nature of the documents to be discovered, but in the case of these TANG memos, the forger appears to have wanted these to be discovered as forgeries.  Anyone making these forgeries would obviously find a typewriter like the ones originally used by the TANG in '72.  The only reason not to use a genuine typewriter is if one wanted documents to be revealed as forgeries, or if one were incredibly stupid.  

Then there's the way these docs were acquired from some dude at a Houston rodeo, and then the tip off to CBS, and then the charges of forgery by the republican operative posing as a Freeper, it's all so...Rovian.  

At this point I'd bet my left nut that this whole forgery debacle is the work of Karl Rove.