This invertible root had a long history in Tolkien’s writings. Its earliest iteration was in a pair of roots in the Qenya Lexicon of the 1910s: ᴱ√OŘO [OÐO] with derivatives having to do with the “dawn”, and ᴱ√ORO “steepness, rising” with which it was much confused (QL/70). The latter had derivatives like ᴱQ. orme “summit, crest, hilltop” and ᴱQ. orto- “raise” (QL/70), and Tolkien mentioned an inverted variant ᴱ√RŌ or ᴱ√ROHO with derivatives like ᴱQ. róna- “arise, rise, ascend” (QL/80). The contemporaneous Gnomish Lexicon also gave it as in an invertible root ᴱ√rō-, oro with derivatives like G. oros “rising” and G. ront “high, steep” (GL/63, 66).
The root reappeared in The Etymologies of the 1930s as ᴹ√ORO “up, rise, high” and ᴹ√RŌ “rise” (Ety/ORO, RŌ). The root was mentioned very frequently in his writings from the 1930s, 40s, 50s and 60s, generally glossed “rise” or “up(wards)”. Thus the root was very well established in Tolkien’s mind.
This root first appeared as ᴹ√RIS “slash, rip” in The Etymologies of the 1930s with the derivative N. risto “rend, rip” (Ety/RIS¹). Tolkien then created a new entry for ᴹ√RIS without deleting the prior entry, with derivatives like ᴹQ. rista/N. rhest “a cut” and ᴹQ. rista-/N. rhista- “cut” (Ety/RIS²); this seems to reflect a conceptual shift of “slash, rip” >> “cut”. Indeed, the root √RIS reappeared in Tolkien’s later writings with the gloss “cut” (PE17/87). Christopher Tolkien gave this root the gloss “cleave” in the Silmarillion Appendix (SA/ris).