Vintage MONO Cartridge Question ?

I take it you want a cartridge that will play pre-1962 mono records in their native mode--in other words, not a cartridge designed to track both mono and stereo records. In other words, with only two signal pinouts (+ and -), and cantilever compliance only in the horizontal plane.

The Denon DL-102 has an internal mono coil arrangement and just two signal pins--one positive and one negative to create a single output channel. It's a high output (3 mV) moving coil cartridge. It is compliant in both planes, but is internally and externally wired to only pick up horizontal motion. I found this description/review on Dagogo enlightening. 

Also, I stand by my recommendation for the Audio Technica AT-MONO3/LP. I got it for playing my 2014 Beatles mono records, but I've found it does well on pre-1962 mono LPs as well. It is dead-quiet in the vertical plane, and makes old mono LPs sound really quiet. Such mono LPs sound unlistenably noisy with a stereo cart.

I am willing to roll the dice with a vintage cartridge.  I have done well before in seeking out 30-40yr old MM stereo cartridges.  The cost of a current production True Mono cartridge starts at about $800-$900.   The cost of acquiring a vintage cartridge, and then having it rebuilt should be about $500-$600.   The assumption being that the cartridge in question does not have a shorted coil, and measures within a few ohms of specification.  The rest- cantilever, stylus, suspension, bushings, can be replaced.   There are at least 2 rebuilders with significant experience in restoring these cartridges, so I am confident of success.

The change from mono to stereo pressings included a change in groove width and shape.  Older mono pressings (pre stereo) can be played using stereo cartridges, but something is lost.  Using a true mono cartridge (that responds to lateral signal only), with a stylus of appropriate width results in much better reproduction than a modern stereo cartridge adapted for mono, and using a current stylus profile.

I'm not a Luddite but I realized a long time ago that "new" does not automatically mean "better".

My system is a combination of new, old and vintage components:
  
Triangle Titus 202 speakers  (10yrs old)
ARC D70 MKII
Cary 40m/SE
Audible Illusions M3
Linn LP12/ ITTOK LVII / AT 33MLocc / Lundahl SUT
Blue Sound Node 2 media streamer connected to an NAS (purchased this year)
AR3a used as subwoofers
Mac 2100 as amp for subs
Mirage powered crossover

Technics SP15 TT / AT 1503 MKII arm to be used a  mono TT

Computer system:
Musical Fidelity a3.2 integrated
Spica TC50
HRT Streamer
AR ES1/ Shure V15Vmr

I also have a large collection of vintage tube equpment:
Heathkit (W4, W5, AA111, UA2 amps)
Eico (HF14, HF86 HF87 HF81)
Fisher (500s, 500c, 500b, 400, 600, X202, 20a, 30a, 440a. FM202T, FM90r, FM200 + mpx200)
Pilot (AA902 and console stereo amp)
Dynaco (too many ST70 and PAS3, several MK4)
AR (3a, 5, and multiple prs of 2ax, at least 2 XA tables)
Janszen (400z, 600z)
Conrad Johson PV5
Hafler DH101
Several NAD receivers
NAD 3020

I have looked at the Denon 102 and may pursue it as a backup if my effort to restore a vintage cartridge proves difficult.   I have listened to comparisons between the 102 and other cartridges.  The 102 does some things very well- mono image is corporeal and surface noise is reduced, but it lacks the air and HF extension of better mono cartridges.  And there is always the GE VR-II....

I don't think anyone would argue with you that using a mono cartridge to play a mono LP is superior to using a stereo cartridge to pay a mono LP.    And no one would argue with you about WHY this is true.  I also agree with you in principle that I would rather use a true mono cartridge (one that cannot respond electrically to vertical displacement of the stylus) than a stereo cartridge wherein the two channels have been bridged internally so as to produce a mono output signal.  Some might argue that using a preamplifier with a mono switch is identical in effect to using a bridged stereo cartridge to produce mono, but that's another can of worms.

Anyway, all I can do is to suggest you investigate modern true mono cartridges like the Miyajima, EMT, and Ortofon products.

A mono cartridge whose cantilever can deflect vertically can still be "true mono" if it creates no signal in that vertical motion. Such cartridges have the stationery coils aligned to respond only to vertical motion. The vertical compliance protects your stereo records from a brain fart when one might forget to swap cartridges before playing a stereo record.

And yeah, I know, the OP plans to have a dedicated mono rig where that "can't" happen. Still, just because a mono cartridge has 4 output pins doesn't mean it's internally a stereo cartridge with strapped outputs. Besides, how would that work? Wouldn't you need diodes or a circuit similar to what's in a mixer to prevent the two cartridge channels from feeding back to each other and creating noise? 

What such a cartridge *can* easily do is be an internal Y-adapter so the mono signal is sent to the left and right signal pins so you have true plug'n'play mono to your sterero outfit without need of an external Y-adapter to send the mono signal to both stereo channels.

BTW, from what I've read, the DL-102 may have a more limited bandwidth because its output is pretty high (3 mV) for a MC. This makes the coils on the cantilever bigger and heavier.

OTOH, that AT-3MONO/LP cart is HOMC with 1.2mV output, plenty for  my MM input. It has a conical stylus, which (in my experience) is evidently wide enough (.60 mil) to work well with mono pressings on both sides of 1962.

If you want a fancier Audio Technica, there's the AT33MONO, which is an LOMC (0.35mV) with slightly wider (.65mil) conical stylus.