Strange device identification

The device is indeed a vintage resistive mains adapter plug:

http://www.vintage-radio.net/forum/showthread.php?t=127072

As you suggested, it is used to drop 220V mains down to 110V for older equipment.

Fairly wild guess here, but one possible use would be for old vacuum tube radios that used a series resistor to drop the mains voltage to a suitable level for the series string of filaments. Assuming 735R is the resistance, that would be a plausible value.

Old radios sometimes used an internal resistor, an internal "ballast tube", some of which bear a resemblance to the object in question, or a resistive line cord. The latter proved something of a fire hazard as people did not always follow directions and routed the cord under rugs or balled them up.

The device shown could have been used with such a radio that had its main cord replaced with a non-resistive type.