TT,
This is just my opinion.
I'm sure if you go back through the lines from certain kennels you will find different types including a Glen and others.
There is Fell (A John Park bred dog called Fury), English Bull and Staff (Many times), White lakeland (it is claimed in the famous Tip)
Back in the 70's men will remember Pedigree Wire and Smooth Fox terriers that made their way into working kennels.
If they worked well they were kept, simple as that.
These are not pedigree dogs like some modern terriers.
Outcrossing to a working dog that may not have the same colour or coat type i