I've been looking at those grave registers - trying to learn a bit more about them.
Grave Register Grave 3902 where William A and Ernest S Smith are buried
https://www.familysearch.org/ark:/61903/3:1:3QHV-P3VC-C9WR-F?i=659&cat=825084There is no number recorded in the column for' If Purchased, number in register of Purchased Grave' but there is a scribble (that appears on other graves) that seems to say 'out' ? I really can't decide what it says or what it may mean.
Grave Register entry for Grave 3973, where Catherine, William Watson and Elizabeth are buried
https://www.familysearch.org/ark:/61903/3:1:3QHV-P3VC-C9HK-T?i=594&cat=825084It also gives the dates of burial, so you may want to check those death years for these people as Catherine was buried in 1935 which seems slightly early if as you say she died a year later in 1936
:-).
Though there seems to be something written in the If Purchased Column, there is also a similar scribble relating to the grave.
Have you ever visited Saltwell? Do you know if there is a headstone for these people ? That's a good indication that they paid for the plot as I don't believe that you were allowed to erect a headstone on a public grave.
Interesting stuff, I do enjoy a new database to poke around and find out about!
Boo