Question: when did the nfl switch from leather helmets
[{'answer': 'by 1950', 'provenance': [{'wikipedia_id': '2600884', 'title': 'Football helmet', 'start_paragraph_id': 8, 'start_character': 882, 'end_paragraph_id': 8, 'end_character': 889, 'bleu_score': 1.0, 'section': 'Section::::History.:Early years.\n'}]}, {'provenance': [{'wikipedia_id': '2600884', 'title': 'Football helmet', 'start_paragraph_id': 8, 'start_character': 0, 'end_paragraph_id': 8, 'end_character': 945, 'bleu_score': 0.9872197907830691, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14381793323027371587}, 'section': 'Section::::History.:Early years.\n'}]}]