That panel is prob 20 years older then any hold down requirement + that is a factory installed main if it's UL listed the lack of a hold down is irrelevant . The white PVC is a pet peeve of mine, ever since I cut into a group of lines that was assumed to be sprinkler lines & one was found to have a 14/2 UF cable in it.(power was off & there was no way to tell it was there).