I cannot imagine, without a definitive recipe being found, and considering it was made by thousands of producers across the empire, it would be much different than Thai Nam pla. That is what they have used for untold centuries. Basic garam, from what I have read, is similar. Layer in small fish like sardines, layer of salt, layer of fish, salt, repeat until full. Then put weight on lid and wait for fermentation, strain, and use.
Please, I am curious, get a good bottle of Nam pla and let me know if it is similar.