The chart on the left is an extract of a similar chart I used in my post on the Rusbase USSR Championships. I chose the first year (1971) arbitrarily, because most preceding years showed the same format. The last year (1991) was the 58th and last of the Soviet championships. They event became the Russian championship in 1992.
As for the other columns, the column headed 'CH' is the annual USSR championship and 'SF' shows the number of semifinal events leading to that championship, but what do the columns headed '1L' and 'SL' represent? Rusbase defines these as '1 League' and 'Selected Tournament', but that terminology is not self-explanatory. Furthermore, why do the semifinals stop in 1973, then resume for a few years in 1980? And why do the '1L's jump from 1 to 2 in 1984.
I set out to answer these questions and started by looking at Rusbase. The first '1L' event -- 1 League of 41 Championship of USSR Tbilisi, 4-28.10.1973 -- shows an 18-player round robin where the first six players were Vaganian, Dzindzichashvili, Vasiukov, Furman, Razuvaev, and Bronstein. The page also gave the additional information 'Average Elo: 2474 <=> Cat: 9 gm = 11.90 m = 8.50'. [As an aside, were the FIDE rules governing ratings and title norms already in effect at that time, or has Rusbase extrapolated current rules to historical data?]
The four 1973 semifinals were all played in June-July, and I worked out that the players who finished in first place were all seeded into the 41st championship -- 'Moscow 2-26.10.1973 (Average Elo: 2558 <=> Cat: 13 gm = 9.52 m = 6.12)' -- which was held at the same time as the 'Tbilisi 4-28.10.1973' event. The players who finished just behind first place in the semifinals were all seeded into Tbilisi, and they accounted for all 18 players except for Bronstein, who appears to have received a special pass into Tbilisi. Vaganian, who finished first at Tbilisi on tiebreak over Dzindzichashvili, played in the '42 Championship of USSR Leningrad 30.11-23.12.1974'. Was he seeded from Tbilisi or is this a coincidence? Dzindzi didn't play in the 42nd championship.
Assuming that '1L' meant 'Premier League', I switched to Google. The first stop was a Wikipedia page 1976 in Chess.
Serving as a warm-up for the main Premier League Soviet Championship in Moscow (see above), the First League Championship is held in Minsk. Remarkably, this 'second-string' event is still stronger than any other national championship and the entry includes Mark Taimanov, Alexander Beliavsky, Lev Alburt, Gennady Kuzmin and Semyon Furman. Making the headlines however, are Iosif Dorfman who wins convincingly with 11½/17, ahead of Vitaly Tseshkovsky, Evgeny Sveshnikov and Nukhim Rashkovsky (all 10/17).
The reference 'see above' refers to the 44th Soviet Championship, won by Karpov, who had become World Championship by default in 1975. Since Rusbase confirmed '1 League of 44 Championship of USSR in Minsk, 1976' and '44 Championship of USSR in Moscow, 1976', I now assumed that '1L' meant 'First League', and changed my search term accordingly. This brought up another Wikipedia page, Josif Dorfman, also with info on 1976.
Dorfman played in several USSR championships. In 1975, he took 13th in Yerevan (43rd URS-ch; Tigran Petrosian won). In 1976, he tied for 5-7th in Moscow (44th URS-ch; Anatoly Karpov won). One of his most emphatic victories occurred in qualification for this championship, at the 1976 First League tournament, where he finished 1½ points clear of the field (+6, =11).
Now I was on the right track. Another Wikipedia page, on Igor Vasilyevich Ivanov, mentioned,
Ivanov did qualify for the 1975 Soviet Championship First League; this event, with several Grandmasters in the field, was still one stage below the top level at that time. [...] He shared first place with Garry Kasparov (future World Champion), in the Soviet Championship qualifying tournament held at Daugavpils, 1978. But for the 64 players, only one place was open to the Soviet Championship Premiere League [MW: note 'Premiere'], and Kasparov won this on tiebreak. Ivanov qualified again for the lesser but still significant First League of 1979.
Rusbase again confirmed that 'Ivanov I.' played in the '1 League of 43 Championship of USSR in Kishinev, 1975', as well as the 1976 Minsk event, won, as we just saw, by Dorfman. More interesting was the mention of Kasparov. Chapter 5 of his book 'Fighting Chess' (1983 edition, with the assistance of Schiller and Wade) is titled 'Top League! : Otborochnii at Daugapils', and starts,
The Otborochnii (Qualifying) Tournament stage of the 46th USSR Championship held in the Latvian town of Daugavpils had 64 grandmasters and masters competing in a 13-round Swiss system event for one place in the Top League (final) Tournament and a further eight players for places in the (semi-final) First League Tournament.
The 15 year old schoolboy, Gary Kasparov [MW: that's how he spelled his first name early in his career], took the giant step into the Top League by virtue of a streak of 5.5 points from six from rounds 2-8 and a superior Bucholz tie-breaker. And this meant into the top echelons of world chess.
Gary's co-winner, Igor Ivanov, who started with two losses and then reeled off six straight wins before being contained by Gary in 40 moves in round 9 [MW: i.e. a draw], had to be content with a place in the First League, shared 14th place there and thus failed to reach the Top League. What such minute differences lead to!
Rusbase confirmed
Now I was on firm ground. I had the meaning of 1L ('First League') and of SL ('Selected Tournament', although 'Selection Tournament' and 'Qualifying Tournament' are better terms); a Russian transliteration for SL ('otborochnii'); two synonyms for the USSR Championship ('Premier League' and 'Top League'); and a better understanding of the qualification process for the Soviet championships. I also had a little more information about a crucial step in Kasparov's career.
As for my other questions -- 'why do the semifinals stop in 1973' and 'why do the 1Ls jump from 1 to 2 in 1984' -- they will have to wait for another day.