Aluminum weighs 165 lbs per cu ft. Brass weighs 534 lbs per cu ft.
Looks to me like a bright engineer at Brooks decided to save some weight. Must have been a fad at the time as many framebuilders did the same thing with head badges.
This, by the way, is why I love British bikes. There are so many ingenious eccentricities, that its practically impossible to finish a proper restoration.
Amir Avitzur
Ramat-Gan, Israel