Hi Rogdcam, Thank you for the reference. The issue is, the stamp if legitimate has a rather high catalog value and has been previously forged (see the issue with the black background as a forged example). If the legitimate overprint is represented in the Russian catalog I found, mine is definately a forgery. Note the serif on the 1 is very different. If there was an alternative setting on the printing device, which is seen on another of the set, it may still be fine.
