This is strictly my opinion. Others may grade their Swede differently (just like others may like blonds, brunettes and redheads more than browns....I can't see how, though!!
A unit disk does not really decrease or increase a Swede's value by itself. A collector will have a different grading system than a shooter, obviously. If we're talking strictly m/96's, there's a few other factors usually considered and weighed heavier. First being the date/place of manufacture, then rifle condition and, of course, the numbers game.
As singleshotcajun hit on, there's the question of authenticity. The stock can be matched to the receiver by checking the numbers in the barrel channel and handguard. That's a first step. But there are disks floating out there that some put on their bare stocks and also, there are unscrupulous individuals trying to scam.
You can widdle down the authenticity of a unit disk by research. A unit disk precariously placed on a beech stock belonging to a 1943 Husqvarna m/38 would sound alarms.
So, all that being said, I wouldn't pay extra just for a unit stock disk. It's a nicety and cool thing to have, but no other special value.
BTW, here's what the disk says:
Rifle number 1096
65th area of the 28th Infantry Regiment (Landstormen)
The soldier that carried the rifle with that unit disk (let's hope it was your rifle!!
) was part of the "Old Guard"...literally. Landstormen were older troops placed in special designated units mainly performing Army Area Defense. He was also one tough cookie. Freezing his cojones off up north in the Vasternorrlands Regement.