To my eye, Roger's picture above shows the shoes with a distinct location for the fixed pivot. These are likely to be the 5867/8 earlier type. Julian's picture shows "floating" shoes with a flat pivot area, which is fitted with steel pad, shaped to allow a small amount of movement for the shoes to float and achieve better drum contact.
As this is a possible improvement, providing the rest of the drum and backplate were unchanged, it would seem to me that the early and later shoes are interchangeable, and if so explains the short supply of earlier shoes. Bit out of my on hands experience here, so perhaps folks more clued up on later brakes can add their thoughts. Do later floating shoes fit earlier full width cast iron hubs as a straight swap? The floating shoes appear to be identical, changing the pad from one end to the other converts the shoe to leading or trailing position, altering the lining positions in relation to the cam and pivot, rather than the two different shoes of the original design.
Swarfy.