The only argon glow lamp I know of is the AR-1 lamp, which was really only used as a small UV/blacklight source with a high purity glass envelope, never as an indicator. If you can indeed strike a discharge in argon with 120V, why was this never incorporated into small scale neon bulb style indicators?

Also, if you added mercury to a normal neon/argon penning mix indicator lamp, wouldn't that make it brighter/more efficient (and blue)? Why was this never done?

I've seen green argon indicator lamps.
Adding mercury would create a bluish halo around the pinkish neon-argon glow.

The only argon glow lamp I know of is the AR-1 lamp, which was really only used as a small UV/blacklight source with a high purity glass envelope, never as an indicator. If you can indeed strike a discharge in argon with 120V, why was this never incorporated into small scale neon bulb style indicators?

Also, if you added mercury to a normal neon/argon penning mix indicator lamp, wouldn't that make it brighter/more efficient (and blue)? Why was this never done?

Because it will be dim. Low pressure Ar discharge emits significant part of its radiation in near-IR, useless for indicator application. UV generating efficiency is also mediocre, so practical phosphor indicator lamps usually have something added, like mercury or xenon. Adding mercury to phosphorless Ar lamp will make it somewhat brighter but not as bright as wanted for an indicator lamp, cause most energy of low pressure Hg discharge is radiated in UVC. Think of what you see in a germicidal lamp. Clear Ar + Hg  are often practiced in sign tubes, BUT you not always need a sign tube to be blindingly bright, instead often the opposite!

Mainly because their efficacy was terribly low and the life extremely short.

There was a fairly wide range of Argon glow lamps (AR-1 up to AR-9, a miniature type which was the UV version of the NE-2).  But due to the poor performance they were really only used in applications where UV output was required - which tends not to be in indicator lamps.

NB we also cannot really even talk of these being argon lamps, because all were in fact argon-nitrogen discharges.  Pure argon (or argon with a tiny percentage of neon, krypton or tritium to aid ignition) have even shorter lives due to excessive gas adsorption on the nickel, or nickel plated steel electrodes, or sputtering.  Despite intensive efforts, improved electrodes were never achieved.  Alloys or plated electrodes with lower gas adsorption suffered excessive degradation by blackening.

The solution to achieve coloured lamps was to switch to a neon-xenon filling combined with green, blue or yellow phosphors.  I believe Philips was first to make those at its Terneuzen plant in Holland, but Osram may have been first with at least the yellow (which it also offered with lacquer coating to produce green).  GE also extensively made the NE-2 Blue and Green models from the 1960s-80s.  These offered the benefits of long life like the traditional neon lamps, combined with much higher efficacy than argon-nitrogen lamps due to the phosphor stimulation method.

These days its not so easy to find the original Philips, Osram, GE or Sylvania neon glow lamps because those producers all gave up in the 1980s with the advent of superior LED technologies.  But the Soviets kept the designs running until much more recently.  You can quite easily find their ТЛЖ, ТЛО ,ТЛЭ, ТЛГ series for sale today.  I quite like those little lamps which were made in an unusually wide range if colours and illustrate the technology rather well.  I am not sure of their precise gasfillings, but suspect neon-argon + tritium and maybe also with a drop of mercury, at least in the -2 versions which have much higher voltage drop and hence power and brightness.

@James Soviet TL* phosphor lamps (except orange TLO lamp which is just a neon lamp with a diffuse coating)  definitely have something + xenon as their fill, due to specific Xe near-IR signature lines. See here: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-250635  https://www.lighting-gallery.net/gallery/displayimage.php?pos=-250636

@RRK
Essentially it is inefficient is what you are saying?

@James
That is really cool, I was totally unaware of any argon-nitrogen lamps other than Ar-1s, I haven't seen any pictures of them here. An Ar-9 would be really cool to see.

So those colored neon lamps are actually neon-xenon? I always thought they would be mercury-argon like a fluorescent tube.

@James Interestingly if these pretty and short living Aerolux and clones figural violet-green lamps are pure Ar or Ar/N2 fill too? I definitely need to probe my lamp with a spectrometer.