The chart on the left is similar to the chart I used in 'Premier League' and 'First League', except it shows the stages of the USSR Championships in their logical order.
The example I worked out for 1978 showed Kasparov qualifying from the year's 'SL' (selection tournament) into the 'CH' (USSR championship, aka 'Premier League'). Igor Ivanov, the runnerup at the 1978 'SL', qualified into that year's '1L' ('First League').
Another example that I worked out for 1973, showed the top player from each 'SF' (semifinal) qualifying into the 'CH', while the next 4-5 players qualified for the '1L'.
According to Rusbase and as shown in the chart, these two qualifying patterns (SL->1L->CH and SF->1L->CH) were used in different formats between 1971 and 1991. A study of each year might show variations in the qualifying cycles, but that goes beyond my objective here.
The year 1973 was the first year a '1L' took place. Was this a response by the Soviet Federation to Spassky's defeat by Fischer in the 1972 World Championship?