Actually, I think there is no fixed type of blades for a coteng. Some are Bugis-types, while others are a form normally found in the Singora (Songkhla) region, like this one on Paul's website. I believe it is called a keris "celok". A few other examples have had bahari/anak alang types (Paul also has one with a bahari blade), and some have N. Malay blades.
http://home1.pacific.net.sg/~dspf/