Matt, Embleton did a revised illustration in the 1997 edition. I have both the 1974 and '97 books. The only difference I notice, in the newer version, is the lack of a "bag" flap on the fur cap.
The plate description says "No picture of the uniform of the other ranks of the 60th survives from earlier than 1768. For this reconstruction we have presumed that grenadiers followed the dress of the line troops closely..."
I think the key word is "presumed".
You peaked my interest in this unit as there is a reenactment group close by that does an impression of the 60th. I did a little homework on them and they "presumed" their impression also.
What you working on BTW?