Machine Doubling Strike characteristic question?

My question regarding Machine Doubling. I understand the shelf like appearance of an MD effect, but what exactly occurs during the strike to produce this line on the staff of the P on this 1944 P Jeff? Possibly a loose die?
Jim
When a man who is honestly mistaken hears the truth, he will either quit being mistaken or cease to be honest....Abraham Lincoln
Patriotism is supporting your country all the time, and your government when it deserves it.....Mark Twain
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Comments
This actually looks like a repunched mint mark, which is fairly common on war nickels.
Looks like an RPM as indicated by your arrow but also has strike doubling that can also be seen on the first U.
That P does look like an RPM.... Cheers, RickO