I've seen several different casting numbers on that style master cylinder but never seen any differences between them. My educated guess was that these Bendix master cylinders either were made at different plants, like Canada and the other in South Bend, or that they got their castings from different suppliers and wanted to keep it clear which castings came from what supplier.
These were used on almost all drum brake Mopars from '67-'70 so they were making lots of them back then.
The date codes were stamped in 2 different locations, which to me suggests Bendix assembled them at different plants. The 809 ones I've seen have a date code in the nose. The 5601s are stamped in the left side of the casting.
Short of a Bendix employee from that era giving more insight, we may never know for sure. So unless further info is uncovered, either is fine to use.
Brake and Equipment warehouse in MN is the best place to restore master cylinders, there is a recent thread here with moparts members talking about their good experiences with them.
