Hallo,
Saw this one, There is no clue about the origin. I thought it could be interesting to show it.
That's a weird one. A KOS cancel from Hamburg (BORGFELDE) Transit?
This is not my image so...we can see a faint receiver from Budapest on the back.
Isolation of Russian's stamps. Previously used on piece and glued on the cover. So no postal mark from the town the cover was sent from.
Best regards
Sylvain