The MKI had many revisions and different circuit board designs, but there are only two versions. MKI and MKII.
The Herbert prototypes were in actuality a pink/red face with no + or - switch.
The general rule of Herbert's is the earlier the production, the tighter the amp.
The later revision VA, VB etc have a bit more gooey sag possible. They can still be dialed in tight though or a simple mod to the backboard can give you the early Herbert attack.