Not sure about the 21 but I think all older Ventures were built pretty much the same. On my 23 the keel cable is one piece of 3/16" SS 7X19 wire that goes from the winch down to the keel. It is attached to the keel by a 3/8" bolt. The bolt goes thru an eye at the end of the wire and then thru the keel. The eye is formed around a thimble and secured with 2 Nicopress terminals.
The deck does have a wood core.
If you haven't discovered them already, here are a couple of Macgregor web sites:
MacGregorSailors.com • Index page
Macgregor/Venture Forum - Message Index
MacGregor Sailing home
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