I have asked about this myself, mostly out of curiosity. I E-mailed Stevenson (no reply), then queried him privately on the BYYB forum. His response:
Perhaps this drawing will give you an idea, at least:
http://bentown.tripod.com/id39.htm
I'm curious as to just how, exactly, it was lowered and secured.
Since I'm laying my keel right now, I'm contemplating how easy it would be to incorporate a fin keel at this stage, and to extend the stern block down for a permanently-mounted electric auxiliary (cannibalized from a 74-pound trolling electric).