Used Paste Up #394 Pair?

I recently saw the following stamp for sale on Hipstamp - a used #394 paste up pair. I was wondering if such a thing was possible. Wouldn't soaking this off the envelope/piece also separate the paste up? Did the bureau use a non-soluble glue for the paste up? I always thought they just used the regular stamp's gum to assemble these. Thanks!

On flat plate coils, paste-up pairs were necessary to make up the roll of coiled stamps. If the perfs. are genuine and the north to south, edge to edge measure correctly, and are parallel, its a good pair. If careful in the process, it could be removed without breaking the pair. I have removed single paste-up successfully from envelope paper. but as yet, haven't attempted to removed any pairs from a cover. I've seen paste-up pairs where the conjoining isn't exactly aligned where the edges are parallel, but this one looks aligned pretty well.

Thanks Al. Just to clarify the process, I' m assuming the perf edge to perf edge comparison would apply to the one stamp on the right of the lower pic (back).
The other stamp would have a "stub" edge w/o perfs and it's other perf edge may not necessarily align perfect with the other. Does this makes sense?

Canyoneer,

I believe Al was referring to two distinct steps in the assessment of authenticity of such a coil.
One being are the perfs genuine? Hard to do without the patient in hand, but do they gauge correctly, is the shape of the holes right, whether the rows of perfs all start in the same exact position (they should not).

Second, whether the cut edges are the correct distance apart and parallel. In this case, the top (graphic north edge) and the bottom (south) edge of each stamp do look parallel on each, but you might want to look at the image with computer software to satisfy yourself.

You do raise a great question about the integrity of the paste-up. I have an imperforate coil paste up pair on cover which I would love to watermark, but one would have to lift the pair off the cover to do so, and I worry about the paste-up failing.
If I recall my reading on the production of these there was a distinct paste supply at hand used to paste up the strips, and then later to paste up whole sheets before cutting, rather than the workers using just the gum on the stamps. But the paste would have also been water soluble I'm sure.