The 7011372 part no for switch kit fits only 1957-60 models. It was superceded by Buick part no 1179345. The kit for late '55 - '56 Carter series 40 2-bbl carbs is 1172257.
If just the starter switch terminal cap is bad, that was available separately as no 1346587. This part crossed over into some models from earlier years. See attached pages from the 1963 Buick Master Chassis Parts Book.
Nothing turned up from Googling the part numbers. One source to try is www.thecarburetorshop.com
What has been, can be again. (Bob Wills, 1942)