I thought I read somewhere that the Postmaster at the time had visited Germany (pre-WWII) and liked this type of cancel and had one made up at his own expense. (I believe it's a roller-type cancel.)
Now I have nothing to confirm that story, so it may not be accurate.
I guess as long as it served the purpose of a cancel with date, etc., the USPOD accepted its use, even though it was a bit odd looking for a US cancel.
In fact, here's another similar cover sale at a buy price of $1.25, so it doesn't appear as though the type of cancel makes the cover particularly scarce or rare; just unusual:
http://www.discountcoverstore.com/_...ename=A24357