Charles,
A very helpful source to me for very roughly dating some daab is the book I think I told you about at Timonium - Punjabhan,
Silverware in Thailand, Rengorm Publishing, Bangkok (1991) ISBN 9748869563. It has photos of several daab in museums, classified at least generally by era/area. By staring at them long enough I began to pick out the similarities among the Ayutthaya, Ratanakosin, Lanna, etc. styles. Still very general (the Ayutthaya Period covers about 500-600 years, and as Andrew said, the Ratanakosin era starting in the mid-18th century and up to the present day).
A couple things I noticed are that Ayutthaya period daab tended to have larger lotus pommels than Ratanakosin (which sometimes just had a very small bulge at the pommel). Both Ratanakosin & Aytthaya seem to have concave ferrules (sweeping inward), while Lanna style has convex ferrule (sort of bell shaped or bulging outward). Ratankosin daab seem to have quite long handles, Ayutthaya a little less so. The "conch" style pommel seems to appear a lot in the Lanna style. The Lanna daab also have a fairly distinct vegetal decoration, or stylized vegetal decoration (like the one of mine Andrew posted).
But these are just my own crude observations, based on a very few photographs. I learn a little something new every time Puff posts.
This time it was that ferrules running to the end of the blade are rare in Siamese daab, and the Lan Chang lotus style.
