Here's my take. The tunggakan can be found on Palembang keris too. The twisted gonjo can be from anywhere. East coast Malaysia Bugis keris also tend to have a twisted bar gonjo even if the blade is mlumah. The blumbangan looks square - Mataram influenced. The front of gandik is curved - West Java, but many later keris use this feature as well.
So this keris overall style is a Mataram style with West Java influence. The thing about West Java keris is that they usually have a forging flaw somewhere on the blade. But I can't say this for all keris, obviously. Some West Javan keris are very nice kerises. This one appears to have good quality forging. So, I have to vote, I'll lean against Sumatra on this one. Usually the provenance of the keris will be used to determine the final verdict.
Below is a photo of the greneng of my Palembang keris. The keris has a square blumbangan, and the greneng is Surakarta. The blumbangan is cut in the style of this keris uploaded by Anthony.
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