Understanding Society User Support

Hi i am looking to work with the ethnicity data in the BHPS sample, i was going through the data in stata and have found that it is fine until 2003 and after that around 90% of the data is inapplicable in relation to ethnicity. i am currently using bx_racel_bh. I understand that ethnicity is only asked as participants enter the survey and thus a derived variable may be needed but cannot find this dervied variable, am i missing it? any advice would be appreciated.
Thanks