Zadok Domnitz. Photo Credit: Shachmat [Israeli Chess Association's magazine], Year 2 No. 5 (Dec. 1963), p. 6.
The Israeli chess championship of 1961/62 (officially, of the Hebrew year H'TSKV -- 5,722 -- which began in September 1961) caused a sensation: the three winners (with 9.5/15) included not only IM Yosef Porat (who played in all previous championships, from 1935 on, only once finishing below 3rd place) and Itzchak Aloni (a player in the championships since 1945 -- which he won -- finishing below 3rd only twice), but the 28-year-old Zadok Domnitz. Not only did he not have any FIDE title, he was not even an Israeli Chess Association master -- merely a 'senior candidate master'. The other participants were mostly ICA masters, including two FIDE IMs (Porat and Moshe Czerniak).
Domnitz is probably one of the lowest-rated players to ever win a national championship, at least in countries where chess is popular. His victory caused quite a sensation, although Aloni eventually won the championship on the three-way tie-breaking match (scoring 2 /2). (To avoid a possible misunderstanding, 'low rated' does not imply 'weak'!)