The following is from the Audiotools.com site.
Pickups and stylii for the playback of Mono LP records
Modern stereo records have use a groove that is about 18µm, earlier mono microgroove records however had a groove width of around 25µm (usually slightly more than this or up-to 28µm), the difference is small enough to mean that you can usually play a mono LP with a modern stylus but for optimal results it's best to have a stylus especially cut for mono lp's and a true mono pickup (not rewired or a plain stereo pickup) and indeed a few companies make such pickups and stylii. Ortofon has the OM 10 Super repackaged with a 25µm stylus for the playback of mono LP's as the OM D25M, note that this is still a stereo cart and that this stylus is available separatly and will fit all OM, OM Super, OMP, TM and Concorde type pickups from the company. There are also SPU variants for this purpose and those are true mono pickups, namely the SPU Mono A, the SPU Mono GM, the slightly cheaper Classic SPU Mono CG25Di and the Classic SPU Mono CA25Di. Audio Technica has the AT-Mono 3/LP, it's a variant of the AT-33 like the 3/SP but has a full frequency range. Lyra offers the Helicon Mono MC pickup with a stylus for Mono LP's, it's a true mono unit and My Sonic Lab has the My Sonic Lab Eminent Solo which is a true mono MC unit as well. Expert Stylus has 25µ stylii that can be glued onto any cart (I belive they have variants for used with worn mono LP's as well). Note that while Grado offers at the least 2 mono pickups these use 10µm stylii and are thus usable with mono records but not optimal like the pickups here above, these are also not true mono pickups but rather stereo models wired in series.
Pickups and stylii for the playback of Mono LP records
Modern stereo records have use a groove that is about 18µm, earlier mono microgroove records however had a groove width of around 25µm (usually slightly more than this or up-to 28µm), the difference is small enough to mean that you can usually play a mono LP with a modern stylus but for optimal results it's best to have a stylus especially cut for mono lp's and a true mono pickup (not rewired or a plain stereo pickup) and indeed a few companies make such pickups and stylii. Ortofon has the OM 10 Super repackaged with a 25µm stylus for the playback of mono LP's as the OM D25M, note that this is still a stereo cart and that this stylus is available separatly and will fit all OM, OM Super, OMP, TM and Concorde type pickups from the company. There are also SPU variants for this purpose and those are true mono pickups, namely the SPU Mono A, the SPU Mono GM, the slightly cheaper Classic SPU Mono CG25Di and the Classic SPU Mono CA25Di. Audio Technica has the AT-Mono 3/LP, it's a variant of the AT-33 like the 3/SP but has a full frequency range. Lyra offers the Helicon Mono MC pickup with a stylus for Mono LP's, it's a true mono unit and My Sonic Lab has the My Sonic Lab Eminent Solo which is a true mono MC unit as well. Expert Stylus has 25µ stylii that can be glued onto any cart (I belive they have variants for used with worn mono LP's as well). Note that while Grado offers at the least 2 mono pickups these use 10µm stylii and are thus usable with mono records but not optimal like the pickups here above, these are also not true mono pickups but rather stereo models wired in series.