A breakdown of why i think the collagen portion of SP-D is involved in the central area.  The ineup on RCSB shows each AA of SP-D and interesting points along the early part of the collagen like domain which lie pretty much in a line with the point where the arms of the SP-D dodecamers branch.  Also the length of the AA sequence (which hopefully is more or less relative in size for the straight-ish trimer arms and the coiled coil neck, but it is easy to see that it is quite a bit truncated, thicker, and rounded at the  CRD end (and therefore shorter). Red is a list of AA of the Nterminal, black is list of the AA of the collagen-like domain, green is AA list of the neck, and blue is AA list of the CRD.  It is easy to see that the 50 nm (half the 100 nm that is typically used as the measure of an SP-D dodecamer, is not quite enough here.  This might be just because of variation in how this particular molecule is splayed out, or it may be a variation in the attributed 100nm bar marker provided by the author, or SP-D may actually be just a little larger than 100nm.  But what is pretty clear is that there is some of the first part of the collagen like domain that is connected, as a continuation of the center beyond what the AAs for the N terminal would suggest.

Image links to the page of the RCSB that I have screen printed here. And the diagram of the collagen-like portion is a figure from Sorensen, G Frontiers in Medicine 2018.