About 10 years ago a friend bought an old Pentax 67, don't know which model, but it was old then.
He eventually purchased an aftermarket screen. I don't know which one, but it had a grid etched for compositional aid.
I have a Gossen Profi-six meter and one of the attachments I have is the fibre optic measuring unit. This allows one to use the Profi-six, to get TTL readings on the groundglass of a LF camera.
We tested his old focusing screen against the new one, the difference was approximately 1 and 1/3 of a stop brighter in the centre of the focusing screen.
I cannot be sure, but I think it was a Beattie screen.
At the time, I remember thinking I should get one for my Nikon F3, but the F3 screen was double the price of my FE2 screen, I couldn't think of any logical reason for this, so I didn't get one.
Mick.