Hi Detlef,
I would totally agree with the Capn's perceptive observations, and this is an extremely attractive stiletto with distinctly Masonic potential. As noted, the cross guard vestigially recalls the crossed bones which appear as such on the daggers in Masonic regalia with skull pommels.
These are part of the Masonic symbolism known as 'memento mori', which of course is a reminder of man's potential mortality, with skulls and crossed bones being symbolic.
In the latter quarter of the 19th century, there seems to have been a propensity of using bone in hilts, and here in the US, it appears that military officers swords often became host weapons. I once had a M1840 staff officers sword which was most unusual as the standard brass wire grip was replaced with a horn grip. As it was decades ago, and not knowing what it was, I unfortunately let it go
Looking back, I can now see it in this post Civil War fraternal context, much as I agree this stiletto probably is.
Adding to the likely Masonic context here, are the initials engraved in the pommel cap.
While Masonic weapons have typically been overlooked by arms collectors, since the publication of the book "American Fraternal Swords" by John D. Hamiliton, Joseph Marino and James Kaplan, that has quite changed.
The skull and bones theme seems to have been quite well used in French Masonic context as well, as seen in this bizarre sword with full gauntlet style hilt in skull shape, mounted with Napoleonic cuirassier sword blade.
Most attractive dagger with intriguing Masonic potential!