I am definately no expert on this and perhaps Rob would care to comment. From the referenced article the total ballast is 544. I would think this includes the cast iron centre board plus the ballast in the hull. Somewhere I have read that the centre board is around 400kgs. That would leave the ballast in the hull at 144kgs. Although some boats were built to charter specs (mine included) and I think these had an extra 100kgs in the hull.
Feel free to correct me as I am attempting to sell my RL28 and would like to have these specs accurate.
The RL28 swing keels weight was 250Kg approx, and in normal production boats there was an additional 100 Kg of internal ballast added below floor level.
Boats built under survey had 200Kg of additional internal ballast and those built to operate in some exposed waters up to 300Kg was added. In all cases this internal weight became a structual part of of the hull making it very strong, but, also making it difficult to remove.