Go back nearly a decade-SA Carbon ran in early 2015 or so. Alphas would look like Umka, mods were dark grey w orange text-I made a facetious post above about accents basically suggesting that GMK Carbon be made...which was run in 2016, and rerun as an R2 in 2018. For a coupla reasons, R2 didn't sell well; split up alphas and mods get to be expensive. One of the priciest kits per key was an Umka mods set, which let you pair w your Carbon alphas to make a kit that looked like this, but had R0 and R5. Yes it was 160$. Here's the kits that ran, and their prices.
https://matrixzj.github.io/docs/gmk-keycaps/Carbon-R2/So people liked the mono looks but didnt buy them. T0mb3ry made boneyard (which was also the name of the standalone alphas kit from Carbon R2 GB like Umka mods were) and included some new novs, and would be a cheaper way to get the mono charcoal look, just with R1-R4 instead of the R0-R5 profile. GMK Umka - this set - came after. There was another alt alphas kit in Carbon R2-all orange, called Beezare, but that (sadly) never had a base kit run. Emir liked the all grey kit of Carbon 2 as well, and to cut prices/set it apart/colors looked nice...got stock colors N9 and u9, and GMK Nines was born.
TL;DR The set is the mono version of a really classic GMK set; which after 6 years and...a lotta caps, is still my favorite.