This 1964 acknowledgement of the importance of "concealed households" (or MHU, central to my recent paper with Jens), comes from the same piece where Glass coined "gentrification" as a term. Also, maybe Glass knew something about coining a good term? ("Concealed Household" > "MHU"?)
Linked to footnote 3 with some more details and stats. -- Good stuff, I need to make a trip to the library to read the rest.