Vintage MONO Cartridge Question ?

A mono cartridge can have vertical compliance.  The Denon 102, and many of the Japanese mono cartridges have vertical compliance.  What is important is that the cartridge contains a single  coil only, and does not generate signal from vertical movement (well there is some, but it is negligible).  A stereo cartridge, adapted for mono by coil alignment, or internal connection generates a vertical signal.  This signal introduces distortion and phase anomalies that are audible.  
 

I don't really care if the specs allow for some vertical crosstalk. When I lower the needle of my *stereo* cartridge (using the damped cueing lever), there is a loud THUMP! When I lower the needle with my AT-MONO3/LP, its landing is TOTALLY INAUDIBLE. If it's inaudible during a severe vertical modulation with no horizontal modulation to obscure it, then it's not going to contribute anything of note when all the groove modulations are in the horizontal plane.  

It's a really good cartridge for a ridiculous price, and the AT33MONO is even better, as it should be.

In theory, the noise generated by vertical motion of the stylus in a mono cartridge is cancelled by adding the positive and negative signals from each channel, in a mono cartridge that was created from a stereo cartridge.  As I think Fleib is trying to say, since the two channels are never a perfect match, the cancellation effect also can never be absolutely perfect.  The same applies to using the mono switch, if your preamplifier has such.  I don't own any mono cartridge, but I do use a mono switch when playing mono LPs, and the improvement vs playing mono LPs in stereo is enormous, most notably in the area of reduced noise and an increase in the width of the soundstage.  From this I conclude that faux mono cartridges might also work pretty well to generate a mono experience, albeit not perfectly.  Anyway, the OP is not asking us to debate this issue.  He knows what he wants.

A true mono cartridge, which has only one coil, can have vertical compliance without generating any noise as a result, if there is no mechanism to transduce vertical movement into an audio signal.

Another issue, I much prefer 2-channel mono over the idea of using only one of two speakers when listening to mono.  Old habits die hard, I guess.  But this is why I personally have no interest in ancient true mono, 2-pin cartridges from the 50s.  (While I am a big proponent of vintage stereo cartridges.)

A true mono cartridge, which has only one coil, can have vertical compliance without generating any noise as a result, if there is no mechanism to transduce vertical movement into an audio signal.

My point exactly. Vertical compliance by itself does not disqualify a cartridge from being "true mono."

Another issue, I much prefer 2-channel mono over the idea of using only one of two speakers when listening to mono. Old habits die hard, I guess. But this is why I personally have no interest in ancient true mono, 2-pin cartridges from the 50s. (While I am a big proponent of vintage stereo cartridges.)

No argument here! I *love* mono played through two speakers, esp. with my Maggie dipoles which energize the entire room. It's also a great way for tuning the system. If your mono signal is dead center between the speakers and you get a sense of depth, you're on the right track.