My impressions: Tolex looks right, corners look right, internal construction looks right, handle, casters, so yes, probably a Sunn. But the cabinet doesn't appear to be ported in any way, and offhand, I can't think of any early Sunn cabinets that were sealed. L. D. would be the initials of the worker who finished the cabinet. The cutout on the back of the cabinet looks about right to hold a JBL crossover, which would mean that this cabinet would originally have more likely been a PA bottom than a bass guitar cabinet, but, again, I'd expect it to be ported.
Sorry I can't be of more help.