for a 35 year period 50mm considered the standard focal length for 35mm film, which is why there are more listings of 50mm vintage primes for sale on the (E)vil-(BAY) than any other focal length. the second is probably 58mm due to teh ruski's mass producing to surplus the biotar copy known as the helios 44
also most 50mm's are based on a double gauss design known for low distortion and excellent correction of lateral chromatic aberration and coma (when stopped down in the range f4 to f8) the other most popular design was the 4 element 3 groups design known as the tessar, it has better wide open performance but could only go as wide as f2.8 and was not as popular).
on a unrelated note, canon may hold the world record for most lens manufactured of its own ef system, but i bet Krasnogorsk Mechanical Works (KMZ) have made more lenses over the same period of time full stop