Hi Paulo! I have the same question. My guess is that bicycles per 1930s mostly have right side rear brake but not always do. It varies for different brands but right side was mainstream. For example, right rear brakes are rarely used on most Raleigh bicycles but are frequently found on Humber, BSA, Triumph, Hercules and some off-brands in that period. This arrangement is related to nothing but supply chain?? Since Raleigh consistently use its own unique double sides brake joint lever(I don't know the terminology), it less likely changed over time. One thing I know is that Sturmey Archer continues providing right side brake system as 'standard part' in its catalog until 1940s, which indicts the likelihood of bicycle in that period still use right rear brake. Any other idea?