I think our Thai members will be able to address this better than I, but I won't let that stop me from trying.
"
Dha" is a Burmese term for sword (or knife, depending on the suffix applied). We tend to use it generically to refer to the swords and knives of Burma, Thailand, Laos and Cambodia.
As I understand it, the closest corresponding Thai terms are "
daab", and "
krabi", depending on the type of weapon.
Egerton was probably trying to be specific and accurate as he only refers to Burmese weapons as
dha. The spelling of his Siamese weapon names may simply be a function of translation into English letters. "Dass" could be "daab" and "kassi" could be "krabi".
Egerton's description of a "ngao" sounds like a pole-arm of some sort.