I'm so jealous -- you have another rare keris form which I have been looking for, for years!
This keris is from Java, easily 17th century. Probably from one of the Javanese kingdoms in the Northern coast. The parts are complete and fitting to each other. It's not supposed to have a mendak; instead, the hilt has that brass selut. The blade is definitely Javanese. I think the whole package is 'original'. Congratulations!
The sheath form is a very early type, probably very close to the "root" of the keris sheath "evolutionary tree". You can call it the 'proto-ladrang', with the 'leaves' on both ends not as pronounced as later ladrang forms. And yes, it is very similar to the early coteng sheath posted here. This 'proto-ladrang', you may have noticed, is also very similar to the Balinese ladrang form, which also has muted 'leaves' and similar 'keel' shape (see 1st 2 pics below). Unfortunately, I do not have pictures of some of the older Balinese ladrang forms, which are not as fancy as the one below, and is closer to your keris sheath. It has been said that the Balinese inherited the keris-making traditions of the early Javanese Hindu-Buddhist kingdoms, and is closer to the original early Javanese keris forms than in Java itself.
Your keris sheath is probably the forerunner to the early coteng sheath. I would not find this surprising, considering the widely accepted view that the keris spread out from Java to the rest of the Southeast Asian archipelago. While the keris and sheath forms in Java evolved, so did the keris and sheath forms in the other regions, and they eventually became so distinct that the commonality is hard to spot. However, when one looks at the early sheath forms from each of these region, one starts to find the similarities.
Sometimes, you also see sheaths from other regions, such as Sumatra or even Bugis areas, that are reminiscent of this early Javanese sheath form (see 3rd pic below).
Its an interesting connection to think about.