Complex sentence: The first SLR in the 35mm format was the Soviet Union's "Спорт" ("Sport"), prototyped in 1934, was a very smart design with a 24mm x 36mm frame size, but did not enter the market until 1937.
Split into simpler sentences: The first SLR in the 35mm format was the Soviet Union's "Спорт" ("Sport"). Prototyped in 1934, it was a very smart design with a 24mm x 36mm frame size, but did not enter the market until 1937.