1947 Hobbs of Barbican Clubweight Build and many other old builds

Anyone know what year Brooks started making the swallow?

Short answer: 1936
Extra information: In an article 'Brooks Sporting Saddles' in The Boneshaker, 183 Summer 2010, Alexander von Tutschek wrote 'The Swallow was introduced in 1936 as a new sporting saddle and remained in production into the 1960s'.
The 1937 Books catalogue in the V-CC library records that it was

Designed to assist riders who are anxious to convert every ounce of energy into driving power. The touring "multimiler," no less than the racing man, will profit by the absence of impediment to thigh thrust. The "Swallow's" cut-away lines conduce to coolness. Flap spreading and consequent friction is eliminated.'

I will refrain from commenting on the copy.