In what few notes I could find, you are absolutely right on this appearing to be the so called M1742 hanger, which seems to be as much in question as the also somewhat questionable M1751. As you note, the '1751' does have the bars to the knuckleguard.
It seems odd that this guard would have been removed in this way, again as the M1796 disc hilts were cut down on the inner side in the years after Waterloo. There did seem to be a great deal of concerns in this period about uniform chafing.
While these M1742 hangers have been thought to have been in use during the American Revolution, there have been suggestions that they actually date later, and the debate continues.
Not much help Im afraid, but sure has me curious
All best regards,