Pre Model 5 screw - no H
Pre Model 4 screw - 4" has the H, 6" does not
28-0 - 4 screw - has the H
28-1 - 6" has the H, 4" not found yet
28-2 - has the H (S serial number)
28-3 - no H
I don't currently have a 28-2 with a N serial number so I don't know if they have the H. The above is true for...