Yeah, they certainly could have written stuff in the amp cabs. Thinking back, I'm not sure that it was necessarily the speaker cabs only that they did this to. It could have been any cabs made in the shop.
When I first found Sunn in Tualatin, it was a very unimposing building with a gravel parking lot. I don't think they had much in the way of signage. Tualatin was a very small town about 15 minutes south of Portland. I eventually moved there with my hippy friends. The town soon became a mix of farmers, rockers, and bikers, co-existing. That is all gone now, as the place has become a fairly upscale community of tech workers. Norm Sundholm still lives in the area. In the 70s, you could find tons of Sunn amps and cabs around Portland. Their speaker cabs were incredibly rugged.