Nice original piece Roy.
The discussion so far brings up again the maddening problem of terminology in our highly complex little world. A Malay from Siak might have a completely different understanding of the terms, bahari, anak alang and panjang from that of a Malay from Kelantan. A Kelantanese would call this a keris bahari, our museum's original records sometimes record the term keris alang for similar pieces. The term panjang, which in Malay means "long", clearly ought to relate to a piece of a length greater than the usual, and 13 inches is probably about as average a length as you can get as far as kerises are concerned.
The keris panjang is also often referred to as a keris penyalang or "execution" keris. Of course any keris can be used to execute someone, and often were. The keris panjang/penyalang's role was as much symbolic as anything else. 19th century and later portraits of Malay rulers often show the king holding, or at least having in his presence, a keris panjang/penyalang. The underlying message being, I have the power of life or death over my subjects. And this, while also wearing his personal keris, which had a completely different symbolic message - portraying the king as the leader of his military forces and of his ability to defend himself and his right to make war.
So the short answer...call it a bahari, call it an alang or anak alang I suppose...please don't call it a keris panjang
British scholars of the colonial period would have called this a "Malay" or "Malayan" keris. I think the Dutch did sometimes too ("Maleiche") although they tended to prefer "Sumatra", that is, when they bothered to pay any attention to Malay kerises anyway, which was rather rare, besotted as they were by central Javanese kerises.
However the Malay world and particularly the area surrounding the Malacca Straits was a diverse cosmopolitan cultural complex that includes much of the eastern coast of Sumatra; from at least Deli south to about Jambi, and the western coast of the Malay Peninsula from about Kedah southwards into the Riau Archipelago. By at least the 18th century this area was a jumble of Malay, Minangkabau and Bugis influences...not to mention contemporary Indian and European tastes.
Its pretty difficult then to pin down exactly where this keris comes from. However, I presume by "local" antique shop you meant in Glastonbury? Not a hard and fast rule but it does at least favour the probability then that this piece was a colonial era bring back from Malaya. Also, there are at least a few pieces like this in our museum collection linked to Negeri Sembilan. One or two others are provenanced to Riau or to Sumatra though so...grain of salt.
It is a lovely piece though and I hope you are successful in cleaning it up...do please remeber that in restoration, very often, less is more.