It's hard to effectively distinguish a single 408H (or 408V) from a single 408.
I'm not sure the date is decisive, but it doesn't help the case.
Looking at your photos, I would worry that the bottom edge of the stamp looks rough. As though it was carefully torn from a sheet with the aid of a straightedge, rather than cut during the BEP coil production process to make a horizontal coil. That bottom edge just does not look like any of the imperf H (or V) coils I have, or have looked at recently online.
Those creases, and the associated little bits of damage on the right edge, are interesting. What do you think caused them?
So I agree with the bottom not being a sharp clean cut. But they are parallel and the right height. the ridges I think are from a dispensing machines rollers or guides. Can we fined out if this company used Gov.imperforate coils?
The horizontal creases and aligned edge damage at right would be exactly the type of artifact explained by an affixing machine set a bit too tight as well as the side cuts being so very slightly angled. Without having the cover in front of me, it has all the characteristics of a machine-applied horizontal imperf coil.
Add: and for the bottom edge ... it may have been roughed-up a bit going through the affixer also.