Do we even know what the "M" stands for?
In 1917, the M. Gropper & Sons company was said to consist of Harry Gropper and Abraham S. Gropper. From a patent for a "Liquid Pistol".
Patent number: 1238733
Filing date: Sep 16, 1916
Issue date: Sep 1917
In 1901, the names given for the M. Gropper & Sons company were Harry & Samuel & Abraham.
The Trow (formerly Wilson's) copartnership and corporation directory of New York City, Vol. 49
p. 195
Harry had a son named Meyer. At least there was a young Gropper named Meyer, and Harry was listed as nearest relative on his draft registration from 1918. Meyer was a packer at M. Gropper & Sons.
(click to enlarge)
(So maybe Meyer was Harry's father's name?)
While I'm at it, Gropper appears to have been (re)incorporated in 1923. From the NY Times, 10/4/23, under the headline of "New Incorporations", one of the listings is: