Payment Allocation when paying by bank transfer

Dear HMRC, I am currently working abroad and have recently received a letter confirming that I can pay class 2 national insurance contributions. The statement I have received details the years that I can pay class 2 for, which are the years I have been abroad; but also some years I am able to pay top up class 3 contributions for, from when I was younger and do not have a full year of contributions. I wish to pay the class 2 contributions and not the class 3 ones. My question is, if paying by bank transfer, how can I ensure that the payment is allocated towards the class 2 years and not towards the class 3 years, as I have noticed that the HMRC bank account is the same for receiving class 2 and class 3 payments. Please advise. In addition the statement details the payment date without penalty as 05/04/2025 - can you please confirm what the penalty is for making contributions after that date as it is not detailed anywhere I can find. Many thanks,

Hi,
We would advice you to write in to us to specify which tax years you would like the payments allocated to, Please find our address below:
PT Operations North East England
HMRC
BX9 1AN
Please provide us with the following
•    your name
•    your address
•    your phone number
•    your National Insurance Number
•    what the payment is for 
•    the period/s you are paying for
•    request for a receipt - if this is required
Depending on which tax years you are wishing to pay for, the penalty for missing the deadline date would be you may no longer be able to pay the tax year or you have to pay an increased rate as the rates can change when the tax year ends.