We tend to think of the bobbed style coming from post-war GIs returning from Europe, but this footage is reckoned to be from the thirties – eagle-eyed spotters will recognise a Knucklehead (introduced in 1936) and a Triumph Speed Twin (introduced in 1937) among the competitors – suggesting they just carried on where they left off.
Amazing how a steep hill can provide such simple pleasures: wonder if they did a risk assessment first?