I think the answer to that question will come from Colt advertising and price lists. The A-70 series of catalogs picture no Bankers Special Revolvers (as I recall) and they covered the 1920's until the larger format catalog was introduced in 1929.
Serven says that the Bankers' Special was introduced in 1928.
If one has to choose between two soureces for correctness, and one is TBOCF and the other is a sloppy source like the error-filled Blue Book, I would take TBOCF every time. What difference does a year make anyway?