booster = capper, confederate (antedating HDAS to 1893 April 12)

[Garson O'Toole]

The Historical Dictionary of American Slang has another sense of
booster as a confederate in a scam (page 241).

booster n. Gamb. 1. SHILL; CAPPER. The first citation is in 1906. OED
does not list this sense for booster. Here is an example in 1893.

Cite: 1893 April 12, The Day, No Chance to Win: Gambling Devices Which
Turn All Money to the "House", Page 7, Column 4, New London,
Connecticut. (Google News Archive)

He always has two or three, or perhaps even a half-dozen, accomplices
out in the crowd who win with great regularity. These are the
"cappers" or "boosters." They and the operator have a language known
only to themselves.

http://goo.gl/5x4TQ
http://news.google.com/newspapers?id=txJHAAAAIBAJ&sjid=ffgMAAAAIBAJ&pg=4086,7322919&dq=booster-steps-forward&hl=en

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