What kind of equipment did this guy use?
Look for yourself. It’s right there for you and everyone to see. 
They lose weight mostly by dehydration. The idea is dehydrate, make a lower weight class, and then re-hydrate and you can compete at a higher natural weight, where you are bigger than your opponents.
Often wrestlers at least will try to keep their bodyweight close to a weight limit. That way, they can go up or down a class depending on the strategy of their coach in a team meet.
Strong abs are an advantage. Visible abs are not, except in bodybuilding. Often those with visible abs have naturally low levels of bodyfat. And the restricted diet necessary to produce visible abs is often bad for athletic performance, and with no payoff - nobody cares how buff you are in competitions other than bodybuilding.
Regards,
Shodan
Lot’s of people has washboard abs back then, all it took was access to the right equipment, the proper diet, a little peer pressure and teamwork!
The “washboard” look is not uncommon in males from foraging societies, even though they don’t perform any special exercises to develop them. Take a look at these indigenous Australians:
