Rasjid
Have a look at image 12 for the gonjo.
http://www.kerisattosanaji.com/INTERPRETATIONPAGE3.html
Image 11 shows a keris that seems to be between a KB and a recognisably Modern Keris. The gonjo of this old waved keris is KB style, material conforms, but then we've got that long gandhik, and of course the waves. I've opted to call this "transitional", but we could equally call it "early Modern". Maybe the same line of thought could apply to your keris.
Yeah, based upon what you say, I think we could probably think of your keris as a transitional form, the square tang is possibly the decider, if you get that plus a few other things then its a bit hard to say that it is purely Modern in form.
You say the core is not a side to side core. Is it fitted like a "V", so that it only provides cutting edges?
Really Rasjid, in this whole area of KB's and very early keris, we're swimming in strange waters. To my knowledge nobody else has ever looked really closely at these questions. The whole time I've been studying keris the focus has been on art and to learn how to apply the tangguh system in a way that doesn't divorce you too much from the consensus of opinion. I've gone off to one side.