Merchants Exchange, San Francisco, wonderfully bold red oval local express (merchants' collective) marking, matching 'Paid' in small letters and red ms. '40', also black arc 'Paid' (possibly New York), on Oct. 1, 1850 blue folded letter to Waterbury, Conn., family contents, fresh and extremely fine, a very scarce 'gem', this merchant's exchange was a private hall where members could leave letters for delivery to departing steamers, thus acting as a local express