Green’s Dictionary of Slang

(US black) a landlord or landlady.

1936		C. Himes ‘Let Me at the Enemy’ Coll. Stories (1990) 32: Big Mama May, the landprop, [...] lamped the lain when he drew up.
1958		C. Himes Real Cool Killers (1969) 69: She’s the landprop that used to be old cap Murphy’s go-between ’fore he got sent up for taking all them bribes.
1961		C. Himes Pinktoes 18: Mama Meow, the madame, or landprop as they say in that part of the world, conducts a cathouse in a land of black cats.
1971		H.E. Roberts Third Ear n.p.: landprop n. landlord or landlady.