firewall code question

[black69]

I recently purchased a black 69 camaro ss rs redstripe , black vinyl top camaro from its original owner. Its all original, even paint. Its a 396/ 375 hp 4spd car M21 4:10. It has cowl induction and spoilers and it has the silver grill with black rubber bumper. The question: On the fire wall there is B L K C X or its B L K 6 X. What does the C X or 6 X mean? It looks like the B L K ? X was written all at the same time with the same white marker. Very curious if anyone knows.