willysmjeeps.com

If the skid plates were standard, I wonder why mine doesn't have it? The pan is nice and straight showing no signs of trauma or repair. Was it stock on the CJ5's as well?

Also, were there any other skid plates on the truck? I have none...

The wagons and trucks with those 4 cylinders did not come with skid plate on sump. The universal jeeps civvy & military had tranny skid plates.
Have you downloaded the parts manual from our downloads page yet. If you look these parts up in the parts manual it will be obvious what came with and not with the jeep. When these oil pans suffer a serious rock strike or worse they get replaced. If the owner does not specify the pan with a shield then he'll get the unshielded pan.

If the engines were black from the factory why are restored Army Jeeps painted OD?

They were black at Willys on the line and were painted OD when they were serialed for the Military. A correct resto would have painted the block black first then OD.

Could a USN Jeep have come from Canada (blue engine)? Or maybe it's a rebuilt engine? My Jeep s/n is 64580 and the engine s/n is MD 83903.

That would be very unlikely. Blue was a common in the field paint choice for field rebuilt engines. In your case the MD83903 engine is close enough to the jeep's serial to be original to the jeep. If the military ever rebuilt that engine it would have a rebuild plate on the side of the block. With jeeps over 50 years old they could have been rebuilt several times while out in civilian use and never have a rebuild plate. The fact that you can read the serial off your block's serial pad indicates the block was never decked. This is a normal activity on an overhaul. Not always the first overhaul but certainly decked on subsequent overhauls.