Offset Printing Perhaps??

I've been trying to Identify some of the stamps on some of the covers that I have and came across this one.
I believe the stamp to be MR4 Die II and it's placed on top of a Field Post Office hammer then cancelled in Toronto with a slogan cancel reading " HELP TO WIN THE WAR BUY WAR SAVINGS CERTIFICATES " followed by the flag.
However I'm a little puzzeled by the stamping on the back as it's a mirror image.
Would this be caused by offset printing?
If it is offset printing would it be common or unusual?

Cheers, Bill

It was lying on top of another, just cancelled cover?

Peter

Peter, No. Why guess? If this were a case of stacking freshly-canceled mail, then wouldn't it appear on almost every cover?

By this era, canceling machines were well-tuned to cycle once for each mail piece passing through typically based on a tripping mechanism. The mail piece passed along between various feeding belts and rollers to the circular canceling die and the paired roller on the reverse to provide pressure. When the tripping mechanism broke, the machine might cycle continuously and print the cancel onto the back-side roller. Then, when the next piece of mail properly passed through, it would pick up the reversed ink image from the back-roller and print it on the back of the mail piece - exactly as yours shows - evidence of a broken tripping mechanism - probably needing something as simple as a new spring. Interesting, but not of any added value.

Peter is correct
Reserved the back to see it better.

Robert

I know from collecting currency a very similar thing happens when sheets of notes are stacked on top of each other before they have dried properly
This is called offset printing and is very desirable to certain collectors and carries a nice premium.
The explanation from John Becker is interesting and would explain the squished out look.
Looking at how fine the lines are in the proper impression could there be enough ink to leave such a strong impression from just stacking.

Cheers, Bill

My reply is based on 30+ years of collecting machine cancels and owning 4 actual machines, two of which are the same make/model and which don't feed the same due to some mechanical difference I have not tried to diagnose. I am unaware of any machine with belts configured as the above diagram, but that does not change the general premise.

I would contend that a canceling machine IS a type of printing press - just optimized for printing cancels on letters.

Yes, the cancels are printed directly from metal dies. The reversed impression on the back of any envelope is from the set-off from the feed rollers.

Sorry John

I removed my post...I respect you and your knowledge...I don't want to semi-argue with anyone, especially you.

Robert....

LOL, You can argue with me. It just makes my posts look odd when they reply to something which is now absent. Enjoy!