Again, we are making the assumption that this is perf 11-1/2 x 11. The cancel is not easy to read but I think the year date is 98 (1898). This then fits with Zumstein 72D. If the dating is correct, then this cannot be the very rare 72F (which is perf 11-3/4 and a different color).
So go back and re-measure the perfs to check if my guesses are correct.
There are no forgeries that I know of for the Standing Helvetia issues. There is the possibility of reperfing mentioned earlier.
Thank you for persevering with this. All stamps of this period are not so complicated as I'm sure you have found. But these are. And there's worse. Getting the watermark and perfs correct is the important thing if you want proper identification.
Wow!! Thanks a lot on all those information! Now that I understand a bit more and I know what to look for, so I can tell you : 1) For the 50c I can see the oval without the zippo fluid. So I guess it's the 70A as you said. 2) The 3fr is on granite paper (blue and red pieces of thread). I can see two parts of large crosses with the zippo fluid. I think that is 11 on the long side and 11-1/2 on the short side of the stamp (the dents are not in great shape). So as you said, 72D should be the one. 3) About the 30c, white paper. I can see two parts of large crosses with the zippo fluid. With the numbers you gave me, about the perf I can confirm that this is the 88A as you mention.