Help With Russia Identification

Struggling to find this overprint.

Russian Post office in China. Scott #20.

Thanks, how does one tell if it's vertically laid paper ?

thus quoteth Scott

"Closely spaced parallel wires, with cross wires at wider intervals, make up the frames used for what is know as laid paper. A greater thickness of pulp will settle between the wired. The paper, when held to a light, will show alternate light and dark lines."

So for vertically laid paper hold up to light and their should be clear vertical pattern (i think) to the "weave" of the paper.

And since the only 3R50 value overprinted for use in China is #20, the paper must be vertical lined paper on it. (Unless it is a forged overprint, but the base Russian stamps that could have been used actually have higher CV than the China overprint one, at least in the 2010 Scott Classic)