I think the red shield was introduced in 1977 onto the Soldiers and at that time they created the wee panel for the WK stamp.
The WK stamp was not withdrawn until 88
However your knife is a '78, yet has no red shield, it has the WK panel but no WK stamp - so I think it is likely that it was never a Soldier - and that TC81 is correct - in that it is a non-military knife from the factory made up of 'left over' military parts, or possibly even incorrectly manufactured parts - ie panel but no shield?
Sakwiki has some good info on the
Soldier page - particularly the links to the old SOSAK articles.
I am no expert - just regurgitating what I see there!!
But this is a nice find, in its uniqueness, if our theories are correct