That’s a lovely pattern which has gone largely undocumented.
The spoon is by Holmes, Booth & Haydens, founded in Waterbury, CT in 1853. They were a prominent manufacturer of brass and copper goods who, around 1865, began producing flatware. The “Sterling Silver Plate Co.” was one of their divisional sub-brands, used for electroplate (ie “Sterling” is misleading because the flatware was not sterling silver and, in any case electroplating deposits ‘pure’ silver onto base metal, not sterling silver). Flatware production under that name ceased in 1886 when the division was sold to Rogers & Hamilton.
They seem to have produced no more than a handful of patterns, only one of which can be reliably dated (at 1884) since it was also used by C. Rogers & Bros, W.F. Rogers, and the Hartford Silver Plate Co. Other than that, the patterns are unnamed and Davis and Deibel’s “Silver Plated Flatware Patterns" book simply lists your particular pattern as “Sterling Silver Plate Four”.