In Quenya Notes (QN) from 1957 Tolkien gave the root √STOL “helmet” with derivatives like S. thôl or Q. castol of the same meaning (PE17/186). In etymological notes from around 1964 (DD) Tolkien instead gave √ÞOL “stand up, top” as the basis for these “helmet” words (PE17/188).
Quenya
tolu-
verb. stand up, get up, leave one’s seat
pl1. thely or thuil _n. _Q. sōl. >> castol