Kebaya
The fitted blouse worn over a sarong — cut from sheer voile, finished with fine whitework embroidery at the hem and cuffs, fastened with a kerongsang rather than buttons.
A note on dates
This luminous, lace-trimmed kebaya is the version most people now picture, and it's a real garment — just not quite this book's garment. In 1867, Peranakan women in Penang were more likely wearing the baju panjang: a longer, looser jacket in deeper, sober colours over a batik or plaid sarong, cut from cotton rather than voile. The fitted, sheer, European-lace kebaya only became fashionable from around 1900, as Nyonya dress turned toward a more European silhouette.
Part of why the record skews this way: commercial photography barely existed in the Straits Settlements in 1867. Singapore's first studio had already failed for lack of custom more than a decade earlier, and G.R. Lambert & Co — the firm that would photograph the region for the next fifty years — was founded in April 1867 itself. Almost nothing survives to show us exactly what a Penang household was wearing that year. Most of what we now picture as Peranakan dress comes from the 1900s onward, simply because that is when photographs and garments survived to be kept.
We chose to keep the kebaya of the book's imagination rather than the baju panjang of the historical record — it is the more visually vivid of the two, and the one readers already carry in their heads. Consider it a deliberate, acknowledged anachronism, the kind every costume drama makes somewhere. Sarah is a novelist, not a historian, and this is one of the places she knowingly chose story over strict period accuracy.
Sources: Peranakan Fashion and Its International Sources — Roots.gov.sg · Kebaya — Wikipedia · Daguerreotypes to Dry Plates — BiblioAsia