The current design for the one-Baht coin was introduced in the late 1980's. Before that, the coin was about the same size as the current five-Baht coin. You will generally only encounter the really old coins at 'antique' stalls.
The One-Baht coin - Kanchanapisek version.
The Kanchanapisek coin was issued to commemorate the king's 50th anniversary on the throne.
Various old One-Baht coins.
Old one-Baht coins were about the same size as the current five-Baht coin, which led to a bit of confusion when the new coins were introduced.
The reverse side of the coins, shown here, displayed garuda, royal barges and the same view of the Temple of the Emerald Buddha shown on today's coin.