Question: when did air force master sergeant stripes change
[{'answer': '1991', 'provenance': [{'wikipedia_id': '550560', 'title': 'United States Air Force enlisted rank insignia', 'start_paragraph_id': 3, 'start_character': 559, 'end_paragraph_id': 3, 'end_character': 563, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '550560', 'title': 'United States Air Force enlisted rank insignia', 'start_paragraph_id': 14, 'start_character': 0, 'end_paragraph_id': 14, 'end_character': 1054, 'bleu_score': 0.9894455409323081, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8290678258129917267}, 'section': 'Section::::Evolution of the chevrons.\n'}]}]