[alanint]

Vopo is a shortening of the words "volks polizei" or "civil police" as opposed to military police. The civil police were armed with many nazi era firearms right after the war, mostly from captured Russian stocks.
The marks you mention under the barrel are all factory and not import marks. Yours may be an early import. Some of these were actually taken apart to stamp the import mark, an added step that was later dropped. Your import mark may be on the bottom of th rails on the receiver or the frame, as mentioned earlier. It will be a small group of letters. You will usually be able to make out the abreviation for an American state.