Green’s Dictionary of Slang

(US) one’s arms, when embracing a loved one.

1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 329: armstrong heaters — arms when used to hold a girl.
1951		J. West Witch Diggers 184: Dandie put both arms around her. ‘I’m her red-hot Armstrong heater,’ he said.
1988		I. Wallem I Have a Place (1992) 46: After the engine stopped and lights were out, Henry would move a few inches closer and say, ‘How about a little Armstrong Heater?’.