What is a "21cm/mm absorption system" in the context of measuring old photons from quasars? This answer links to Wikipedia's Time-variation of fundamental constants which cites Further Evidence for Cosmological Evolution of the Fine Structure Constant (J. K. Webb et al. 2001), the abstract of which says:


  We describe the results of a search for time variability of the fine structure constant, alpha, using absorption systems in the spectra of distant quasars. Three large optical datasets and two 21cm/mm absorption systems provide four independent samples, spanning 23% to 87% of the age of the universe. Each sample yields a smaller alpha in the past and the optical sample shows a 4-sigma deviation: da/a = -0.72 +/- 0.18 x 10^{-5} over the redshift range 0.5 &lt; z &lt; 3.5. We find no systematic effects which can explain our results. The only potentially significant systematic effects push da/a towards positive values, i.e. our results would become more significant were we to correct for them.


The paper discusses the "21cm/mm absorption systems" further but I don't understand yet because I'm simply not familiar with the kind of work described here.

The text does refer to:


  HI 21cm absorption lines can be compared with molecular transitions detected at mm wavelengths to constrain $g_p \alpha^2 $ ($g_p$ is the proton g-factor).


so I am thinking that "system" refers to a system of transitions rather than a piece of observational equipment. Is it possible to describe the basics and/or point to a source that does?
They have both 21cm and mm-wave observations for those systems.

Caption to Fig.1


  The hollow squares correspond to two 21 cm and molecular absorption systems.


By "system" they are referring to sets of absorption features caused by multiple clouds of material along the line of sight to a single quasar. Each cloud imposes its own set of absorption features. The totality of these absorption features is referred to as a system.