Question: when did state abbreviations change to two letters
[{'answer': 'October 1963', 'provenance': [{'wikipedia_id': '399492', 'title': 'List of U.S. state abbreviations', 'start_paragraph_id': 7, 'start_character': 81, 'end_paragraph_id': 7, 'end_character': 93, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '399492', 'title': 'List of U.S. state abbreviations', 'start_paragraph_id': 7, 'start_character': 0, 'end_paragraph_id': 7, 'end_character': 409, 'bleu_score': 0.9716239638199891, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11902542425230082521}, 'section': 'Section::::History.\n'}]}]