Here is just a "shot in the Dark". Might it have anything to do with the Fredrick and Hart Machince Company that was also located in Poughkeepsie NY during WWII? They made the Underwood slide with the similar "heart" mark, also the M1D flash hider? Hart was head of Munitions Manufacturing Corp. that IBM moved in and took over at the Poughkeepsie plant site.

See here from the Poughkeepsie Newspaper dated October 6 1944....

"Hart Served IBM "

"The decision of the IBM to establish a branch plant in Poughkeepsie was linked with the service to the company of Frederick H. M. Hart of Frederick Hart and Company, inc. of this city. Mr. Hart, over a long period had served the IBM as a consulting engineer. He headed the Munitions Manufacturing corporation, which was a subsidiary of IBM, which was organized March 1941, to establish the South road munitions industry. All stock of the original subsidiary firm was later purchased by IBM and the former subsidiary was dissolved.

Present corporate officials of the IBM are Mr. Watson, president; Frederick W. Nichol, vice president and general manager; Charles A. Kirk, vice president in charge of manufacturing; Roy Stephens, vice president; Louis H. LaMotte, vice president; Ruth Leach, vice president; John G. Phillips, secretary-treasurer; Albert L. Williams, controller; Mary P. Schultz, assistant vice president; Harrison K. Chauncey, assistant secretary; and Garland Briggs, assistant treasurer.

Walter J. Niles is general manager of No. 4 plant here"

Well it could also just be an inspector's stamp too....
Anyway Just my thoughts Thanks
Paul