What documents do I need to provide for marriage verification when holding Kor Ror 2 and Kor Ror 3 in Thailand?

I am in possession of kor ror 2 and kor ror 3.

But I can’t find any papers that we submitted to meet the marriage criteria in Section 6.1, 6.1.3:

‘A copy of letter certifying marital status before registration of marriage (a copy of letter certifying that the applicant is single)

In absence of such a letter, a cop therof can be obtained from the District Office where the marriage was registered’

Marriage was registered at the registration office in Phra Nakhon District 20th January 2003.