Most of the Japanese rifles with completely ground Mum's came out of Japan.
Yes, the US military did have a ground mum policy from October 1945 until some time in late 1946 or early 1947.
The reason for the droping of the policy, was the Japanese Emperor by Imperial edict (a proclamation by the Emperor) declared himself to be a man, not a God. Thus there was no longer a need to save face by removing the Imperial Crest from the rifles.
The Japanese soldiers in the field used what ever means they had to deface the Mum before handing it over to the allies. That ment hand grinders, files, hammers, chisels and when nothing else was available, the bayonet. A Mum with a "X" filed through it is common, as are mum's with a couple hack marks accross the Mum.
If the Mum is damaged in any manor, it is considered a decrested rifle to serious collectors.
As to the dust covers, they were intended for use in dry dusty areas, like China. When not needed, they were removed fron the rifle and stored in the soldier's back pack. No Japanese soldier ever just removed his rifle's dust cover and "threw it away". If one did, and he could not produce his rifle's dust cover during an inspection, he would be beated to a pulp by his NCO's.
Regards, Gregg