The Glossary of Arms and Armor written by George Cameron Stone Page 451 showes a nice picture. He writes "The "dagger of mercy" intended to be trust between the plates of a fallen foe and give him the "coup de grace". The sight of the uplifted dagger often caused him to surrender, hence the name. It had a long narrow blade intended solely for thrusting.(Hewitt I,319)"
The other interesting thing is it would also be thrust through the eye slots in the helmet.
In figure 572 same page the blade profile is a long tapered diamond shape. Standard cross quillion with a wire wrap handle a small ball for the pommel.
Museum Replicas comes to mind as having several daggers shaped like this in there catalog. Most of there stuff is made in India and quality is all over the place.
Chris Poor makes Top Notch Replicas his web site is
www.armor.com. Go to the dagger section and look at the "Itallion Stilleto".
This is very close to what Stone calls the "Misericorde"
Hope this helps.
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There is no man worth a leke,
Be he sturdy, be he meke,
But he bear a basilard.