Although I'm a copper collector its a not series I have studied in detail.
I've just got "good rimmed examples " of both coins.
Matthew Boulton in 1797 was given the contract to produce these coins.
His mint was based in Soho, Birmingham (not the red light Soho of London).
The presses were operated by steam.
The idea behind the cartwheel coinage was to prevent counterfeiting...previous copper was being melted down and reshaped on thin flans (evasions) and these were being widely excepted by the public.
By 1805 (Battle of Trafalgar..Nelson et al) intrinsic metal value had increased to exceed face value by 30% on cartwheels.
The dies were eventually sold to W Taylor who issued his own collectors editions.
I don't think rusty dies caused the varieties.
The ship,drapery,waves,rocks,sea positions and stops were often altered.
In coin fairs in the UK I have noticed specialist collectors searching out the specimens and the price varience is tremendous for the finer examples.
Its a specialist field.....Pecks reference work covers 50 pages for the coinage of 1797.Hope this helps.