Found these two near each other. On impact they seem the same. When I look closer I notice the bottoms are different. One is blank one has a 6. The forms of the bottles seem slightly different. What are the differences about?
They were made in different molds. If they were making a large number of bottles they may have had multiple molds going at the same time, and I imagine they weren't terribly concerned about making sure each mold was identical. That's assuming that the two are the same age, if not it's possible that one mold was a replacement for the other.
Sometimes molds just wear out or get damaged and it's replaced. However, using one mold for mass producing bottles was very impractical and so many more were used. Thus, I'm pretty certain they had more than one mold which may account for the base marking difference.