I ran across this research article in Hortscience February 2019 by the University of Georgia Horticulture department which I have linked to below with free access to the pdf file. By the way, all of Hortscience and HortTechnology issues from number 1 to present are free access for those with time to dig deep into their favorite subjects.
This one is especially interesting as they purchased and grew out various popular heirloom and open pollinated cultivars from different seed companies to see if any variation exists. Companies included Johnny’s, Clifton, Baker Creek, Harris, High Mowing, Sow True Seeds, Seed Savers Exchange and Sustainable Seed Company. Although most of the companies offer the same cultivars, they most likely have different sources to get their seed stock from.
To sum up the 13 page article, there was indeed variation in the same cultivars of many. Popular varieties like Moon & Stars, Charleston Gray and Sugar Baby had the most variation. Less popular like Georgia Rattlesnake showed no variation. Which suggests it came from one foundation source. In the case of Moon & Stars, the USDA maintains their one lot from Seed Savers Exchange. Other sources had variations that were worthy of maintaining to truly represent the pedigree of that cultivar. Their conclusion is “The active maintenance and protection of genetic variation among seed lots may prove essential in the long-term conservation of these beloved heirloom cultivars.”