Oxide content description
The notation, upon replacing potassium with sodium, can refer to a water glass composition. The unique notation, in my opinion, is warranted based on the following comment from a paper1:

Despite of years of investigations, there are still questions about
the molecular species configuration of these solutions that need to be
answered to gain clarity over the best conditions for the various
applications
When adding sodium hydroxide ($\ce{NaOH}$) to a water glass, the ratio (molar
or weight) $\ce{SiO2/Na2O}$ is decreased. This ratio is called water glass
modulus (n) and determines various physical and chemical properties
such as the pH and the viscosity of the solution.
[...]
the viscosity is extremely dependent on the modulus and increases as
the solution becomes either more siliceous or more alkaline, i.e. at
both higher and lower $\ce{SiO2/Na2O}$ molar ratios (n). Since the viscosity
of such disperse systems like this is given by the silicate
conformation i.e. extended chain conformation and the degree of
polymerization
[...]
the degree of polymerization of the predominant silicate species in
the alkali-solution [...]

So we have apparently a weakly specified structure characterized by extended chain conformation and degrees of polymerization with its rather unique notation (seems right to me).
Reference

Helén Jansson, Diana Bernin, and Kerstin Ramser , &quot;Silicate species of water glass and insights for alkali-activated green cement&quot;, AIP Advances 5, 067167 (2015), DOI: 10.1063/1.4923371