But for more than two decades, Mr. Fox, a retired radio host who earns a modest pension, has been enviably situated in an airy abode with park views, burnished wood floors and historic detailing. In fact, he is the sole resident of a 29-room mansion in Jamaica, Queens, constructed before the Declaration of Independence was written — a pre-prewar, so to speak.If paying scandalously low rent for one of the city’s 39,000 remaining rent-controlled apartments is viewed as the holy grail of New York real estate, that is only because so few are aware of the existence of an even more elusive and lustrous prize.