Firstly, please don't believe all what Wikipedia has to say about Top Hats.
I've tried on numerous occasions to correct what is posted there only to have it changed back! [I think by the original poster]
There is anecdotal evidence that the first ballast was solid cast lead [MK0's], this progressed to lead ingots [Mk1's & Mk2's?] and then to steel shot
coated with oil and sealed in. [Mk2's & Mk3's] I do have a couple of the lead ingots and I'm sure I have posted a photo here somewhere.
You may remember a previous post by a member who had gunk [oil?] seeping from his keel and the repairs he did.
He had no evidence of concrete encasement of the steel shot.
As to what is in each Top Hat I think nobody really knows unless they have opened up the top of the keel.
One thing I can say is that Mk2's & 3's have more ballast than Mk 0's & 1's and that
may include the 6 Formit Mk1's.
Formit added the extra ballast to compensate for the loss of water in the keel tank [caused when they lowered the cabin roof and hence the cabin floor to maintain the headroom] and to stiffen the boat for racing.