Some of the closed knuckles such as found in the M715 are designed like the toyota birfield joint and are considered weaker than the spicer u-joint type design. Most people also consider the knuckle itself a weaker design. I don't know if it is actually weaker but if you break the knuckle the axle is trash wheras a open knuckle design the c's are replaceable (not an easy task on the inner but doable).
The M37's i have seen at auctions all had the weird 9" looking front axle in them. The civilian powerwagons with the fixed roof came with the dana 70 closed knuckle design sometimes. The M37 is a 3/4 ton truck and not a 1 1/4 ton.
There is a book available to those that want to restore them to stock.