It appears that we have an answer to the question of what kind of "experimental warheads" were tested in the September 12 Yars launch. And a confirmation of a guess made on Twitter by @artjomh shortly after the test. The image above is from that post - it's from a textbook they use at the Bauman University. According to The Diplomat's Ankit Panda, Russia tested "an independent post-boost vehicle (IPBV) configuration for a three-warhead version [of the Yars ICBM]." The Bauman textbook calls it "an RV with independent (parallel) deployment." As can be seen from the image, each RV is sitting on top of its own mini-bus, which is different from a more common configuration, when one bus deploys RVs one after another.