Sixpence,
You are hitting something I have been trying to prove for a while. The fact is there just weren't that many F4U-4's built with .50cals and the majority of the F4U-4's ever built were delivered before the wars end. In fact F4U-4 production had completely ended by late 1945. It is highly unlikely that no 20 mil units made it into the field.
Two other items.
1. The 20mils and .50 cals were almost interchangable on the F4U. Even in Korea these guns would be interchanged depending on the straffing mission. 50cal was preffered for anti personel and 20 mil for hard targets. And there were many F4U-1D's for just this reason in Korea.
2. When reffered to the F4U-1C and F4U-4B/C are referred to in place of one another in many places in different pubications.
Here is proof of the existance of this mythical A/C. Notice the climb rate.
