In "My Life and Game," Borg (and/or Gene Scott, his co-author) actually described his forehand grip as being semi-western but many today would probably see it as "strong" eastern or "weak" southwestern. I think that over the last several years there has been a subtle shift in how the grips are identified by instructors. The eastern forehand (traditionally found by "shaking hands" with the racquet or running the flat palm of the hand from the string bed to the handle) seems to have remained the same but all the others have moved over about a half-bevel or so from where they used to be taught. For example, we used to refer to the full western as the "frying pan" grip. Lay the racquet flat on the ground, pick it up like a skillet and, viola, western forehand. Now it seems that move would be seen as creating more of a semi-western grip as it doesn't result in the index knuckle being centered on bevel 5. Same with the eastern backhand. Index knuckle on bevel 1, like Federer, used to be considered an "extreme" eastern - now it's the standard.
It appears to me as though these shifts occurred as a result of the eventual acceptance of mid and oversize racquets as the norm. With an old Kramer Pro Staff one could not get too extreme with their grips (moving the plane of the string bed away from being perpendicular to the court) as it would dramatically increase the likelihood of a mis-hit. As such, the change in hand position from eastern forehand to continental to eastern backhand was described as being much more subtle than it is today.