stereo cartridge on mono record

Jaybo...If you had ever listened to the L-R signal when a mono LP is played you would not call the problem "theoretical".

By the way, you might try listening to the same L-R signal from a stereo FM radio broadcast. The L-R signal is separately broadcast as a FM signal between 20 and 40 KHz, demodulated, and mixed with the L+R mono signal to get L and R. This scheme only works well with a very strong RF signal, which is why in weak signal areas FM mono works fine but stereo is lousy.

A stereo cartridge is sensitive to vertical modulation of the groove. A mono record has only horizontal groove modulation, and whatever vertical movement occurs (and this includes turntable rumble) is pure noise, and you don't want it. It is essential to remove sensitivity to vertical modulation by jumpering the two channels together, or using the MONO switch on your preamp (if it has one).

Improved results may also come from using a different (larger) stylus, and these are available for some stereo cartridges.

Samujohn...It isn't "theory". Vertical groove modulation in a mono recording is pure noise. You are saying that you like some noise mixed in with your music.

Try this...
Connect one channel of your cartridge inverted.
Jumper the two channels using a Y interconnect or a MONO switch. (Now you have a cartridge that is insensitive to horizontal groove modulation and sensitive to vertical).
Listen to your mono LP. What you hear is what is mixed with the music when you use a stereo cartridge without a Mono jumper.