That is all correct. Note the use of the word "registered"...

That is all correct. Note the use of the word "registered" there.

If the books close for registration transfers on 21 April, then the registration status at the registry on that day will determine the registration status on the 29th April.

So even if you did a legitimate deal to sell your units to someone else on say, 25th April, the registry would still hold the original holder to be liable.

So you don't have until the 29th the jump of the train. You have until the 21st to jump off, AND to get the disposal of the units registered by the 21st.