I discovered the same surprise with our boat.. In this thread.
We were 10,000lbs over design weight, this was with the truck scales, which are apparently quite accurate. Empty water tanks, full fuel tanks, fairly basic amount of gear, dinghy, outboard, etc.
I, like yourself, looked at the various stats derived from the design specs. So this was a big surprise as well.. (The PO knew the boat was much heavier tho..
no truth in YW listings..)
After talking to people here and other places, apparently this discrepancy is fairly common even today. Our rigger echoed this point of view. I don't remember the xact story. However, he used to sell J boats, and they had a client that was very upset when the actual weight of the boat was some 6 thousand pounds different from the design weight.
So your boat probably really does weigh 55,000lbs with gear..
My total non-expert opinion -- The ballast is probably whatever they put in the spec as the yard could probably accurately put that much ballast in. Given it was made in 1975, the yard may not have had enough precise control to manufacture something to the designers specs.. or the designer was overly ambitious in what sort of displacement they could achieve.