Hi Clive,
Follow the following IMG path to access search helps:
SPRO>Personnel Management>Personnel Administration>Basic Settings>Maintain search helps
Click on the 'included search helps' to see a list of all the search helps. The "N" and "K" are actually PREMN and PREMK respectively.
Under PREMN, it has BEGDA_0002 and ENDDA_0002 (date of birth because that is the start date for a record on IT0002)
Under PREMK, it has BEGDA_0001 and ENDDA_0001 (begin date of IT0001 record - this is not the date of birth)
You can try deleting the fields from here.
Additionally, SAP has provided user exit HRMC_PREM_EXIT_A for search helps. You can also look to make use of that to hide specific fields.
Hope this helps.
Donnie