There's no meaning in the designation LF other than L - cathegory aviation, F - sixth design team. It was designation used from 1947 to early 50s for design team led by Vladimír Štros. This group originated in Rudý Letov factory (renamed in 1947 to Letecké závody, n.p., závod Letňany - "Aviation Factories National Enterprise, division Letňany", then LET - Letecké závody n.p., závod 4 Letňany - "LET - Aviation Factories National Enterprise, division 4 Letňany" and then shortly back to Rudý Letov). In 1949 they were relocated to nearby Aero factory, at the time known as LET - Letecké závody n.p., závod 3 Vysočany. This factory, division of the same enterprise, became in 1953 direct subsidiary of Rudý Letov.
Because this isn't confusing enough, while the LF group sat in Vysočany, prototype of Pionýr (and presumably others) was built in Letňany factory, three pre-series machines in Vysočany and series production ran in Let (sic!) n.p. in Moravian town of Kunovice. From 1954, series production moved to nearby Avia n.p., závod Choceň.
In early 1950s both glider design groups, LF and LG (based in Otrokovice in the závod 7 of LET, later závod 02 of Let n.p.) were dissolved and no other specialised glider design group emerged.
As for the numbers, yes, it seems to be continuation of Letov line.