Yesterday, I wisited a local every summer sunday outdoor flee market, planned to do it for years, but...
Lot of junk, as I see it, but some interesting stuff.  Folding knives with loose blades and broken points, rusty bayonets and swords,  all sorts of odd items.  Awyway, I carried minimal cash, but hoped I would find a treasure anyway.
And, I feel I DID  .
A one-piece, double-edged knife or dagger, maybe a stilletto.  For 30 kroner, almost 6 dollar, I got the chance to save it from a box with other metall stuff.
It may seem like a letter opener, but it definitively got the feel of a weapon.  And extreme quality.

It is almost 28 cm/11`` total, one edge is 16 cm, the other is 11 cm.  The tickness of the blade is 11,9 mm X 3,9 mm.
After its time in the box its edges was slightly less than absolutely perfect, but 2 minutes with diamond hone and leather strop, and both edges shaved my arm.
There is no rust, just a few minor mis-coloured spots.  It has a mark, F V inside a cirkle.
But, what is this, a assasins tool, a military weapon like the salesman thought, or a sivilian weapon from when?  Any guess?
I think I must make a sheath for it.
timann

At first glimpse it appears like a letter opener, but this easily dismissed by its obviously deadly feel and ability, its lenght and extremly good, sharp edges.  Much better than most double edge daggers.  And trust me  , when held it is obvious it is made for killing human beings. I have a little collection of knives, and like to think I know a few things about this.  It may possibly even be made to look like a letter opener, but is not ment for that purpose .
timann

The handle is wery minimalistic, it is the end of the blade simply flattened ond shaped for holding in a forward stabbing grip.  Look simple, but fit cleverly in a palm reinforced grip.
The handle give it that letter opener look, the blade does not.  No wholes for bayonet or other attachment.  No way for me to attach a paracord grip.  It is just stealthy.
And yes, it looks a bit like a surgical instrument,not that I want THAT surgery .
It could have been made by an surgical instrument maker.  The shape, the perfect, practical grind. Double edge knives that thin usually dont reach that grade of sharpness.  The simplicity. They did that in the past, instrument makers. 
My feel of the blade against diamond hone, and the way it resisted abuse in the box where I found it, tells me of good steel.
timann

The handle is hollowed in on on both sides, it appear to simply be made to be easy to hide, and appear to work well, but it is not a super duper fighting dagger handle which the blade appear to deserve.  Appearently stealth, and sudden death, is the main purpose of this knife.  Oddly, it appear to be of GREAT quality.  That is what interested me with it.
timann

yep - didn't realise the handle was all steel. Definitely medical.

Although you could easily see it snugged under a rifle with a clamp holding on to the neck.
But right from the get go it looked like a medical knife to me. The total steel construction pretty much clinches it.

i personaly think its a throwing knife...  longer for deep penatration, welled in handle to fit the thumb in... it being double sided make me think its unlikley for a surgical intriment or autopsy knife (creepy!) and that its nearly a foot long dissmises the idea of surgery to me...

This is indeed a surgical instrument - it's an amputation knife; specifically, a catlin (double edged).  This knife dates reliably after 1880 by the metal handles.  Knives produced before 1880 would have had bone, wood, stag, or similar handles, but after the invention of the autoclave in 1879, knives would typically have steel handles so they could be sterilized.

For the uninitiated an autoclave is a dirty great big pressure cooker used to sterilise scientific glassware and other equipment and medical kit.
Used to be my job to empty the darn thing way back when

Yep an amputation knife would need to be able to cut both ways to get through the cartilage around the joint. Slim and pointy to get into the joint and double edged to cut quickly both ways.
Nice