Yes, in theory self draining. Only 2 plugs, one on each side of the block.
However, sediment (primarily sand) can inhibit draining. This is how I drain mine:
*As flow diagram shows, water comes in via the 2 hoses from the intermediate housing. I disconnect the hoses at the rear of the manifold & pour some antifreeze into the intermediate to chase out any trapped water. You should see antifreeze come out at the water intake on the front of rudder/trim tab.
*With the lower hose disconnected at the front on the manifold, I pour some antifreeze into the rear of the manifold until it comes out the front.
*Water exits the t-stat via the hose that goes to the upper fitting on the front of manifold. Again I pour some antifreeze into the manifold until I see it at the exhaust, just below the ball gears if the unit is tilted up any amount or eventually at the rudder/trim tab if unit is fully down as this is actually the exhaust outlet.
*I also disconnect the big hose at the t-stat and pour some antifreeze into the hose until it drains out the 2 plugs (petcocks) on the block.
It doesn't require much antifreeze, you are not filling anything, just chasing any trapped water out. Probably less that 1/2 gallon is all you'll use.
Someone may disagree with my procedure, maybe saying it's going overboard, I don't care. Remember, you are dealing with 57 year old equipment that went out of production in 1972. Treat it well, as parts are scarce. That engine looks exceptionally clean. Any other pictures you can share?
