dating ampeg svt HD

[belgamrood]

hi there

i am trying to determine the date of an ampeg svt, 4input blackface black grillcloth
on the back it says:

made in usa by SLM st louis
and there a serial stating HD 1091

i am aware that there are the skunkworx versions of this amp, but there supposed to be a brass plate screwed to the back mesh, but also these happen to get lost through the years

this one hasnt

its a full black, slanted (no cut out)

any help any one?
im just trying to figure out if its worth buying for the price this guy is asking