From Dan's review linked above:
"With this unique bottom plate, Yamashita Hobby takes a more proactive approach to keeping a waterline hull flat. Essentially, it enforces hull stiffness by adding a raised, lengthwise ridge along the top of the flat bottom plate. It resembles an upside down “T” in cross-section, along with some bulked-up attachment points along the hull and within the bow. I did not test fit it, but it certainly seems like it could do the job."
To be very clear, this piece is not a waterline plate but a bottom plate as stated. After assembly of the three hull parts, the two sides of the hull completely enclose this third "unique" part; leaving none of it visible. Having built the YH Fubuki, it seems to me that this new method for the hull has overcome the hogging problem their earlier kits, with the one piece hulls, were prone to.
My Mutsuki arrived in yesterday's mail and I had to immediately go beyond fondling the plastic
to assemble the three hull pieces. Very nice; the bow profile is exceptional.
Everybody goes gaga
over whatever Flyhawk does; with some justification. IMHO, with this release Yamashita Hobby is right up there with Flyhawk in terms of fineness of molding and level of detail.