BBM
Not sure where bruising specific to K comes from?
“Mabbutt said that the victims showed "little bit of bruising" but "nothing significant." She added that some bruising "wouldn't be uncommon during a stabbing."”
11/17/22 Coroner Cathy Mabbutt interview
Coroner Has New Details About Fatal Slaying of 4 U of Idaho Students After Autopsies
If you are the right type of person, you can google "forensic knife wound patterns" or similar (or go to scholar.google.com). Regular google has more pictures, of course.
Someone posted a forensic picture here, early on (of a wound inflicted by a specific size Ka-Bar knife, in which the hilt bruise was clearly visible). Naturally, not every single use of a knife is going to leave crisp forensic evidence, but I do believe it's very easy to know the length of the knife and whether it had a hilt, as well as (usually) the approximate size and shape of the hilt. That's why LE went looking for Ka-Bar or Ka-Bar style knives in the first days after the murder (to at least two different stores - it's on the media thread).
Bruising is not uncommon in a stabbing - not having bruising would be unusual, especially with a hilted knife. The point is, that's the very type of thing forensics people use to figure out what knife was used (and LE act as if they have clues on this).
Further, the existence of bruises next to the knife wound says a lot about the force used.
I'm reluctant to post any articles or pictures here, but you can easily find them with various search terms. "Stab Wound Analysis" should turn up an excellent page with the list of things a coroner is supposed to measure about a wound - and it talks about the impression of the hilt on the skin, which along with other things, can often narrow the knife type down to just a few. It's quite a long list of evidence, actually, just from one knife wound.
I've always assumed that since LE went, within just a few days, to local stores inquiring about a particular knife, that the hilt bruises and taper analysis of the wounds gave them a good idea what knife it was.