Revisiting the subject of the Swiss A17 illustration, based on suggestions from Philimpex and Partime, as you have noted from earlier discussions, I have been examining the subject to determine
a final conclusion, as much as is possible. My initial assumption, based on color and impressions leaned towards #21. Upon examination by SCO based on examination and expertise, leaned to #27. Additional examination leaned towards the assumption that it was likely #37.
Criteria for review included higher magnification of the thread on the back and the postmark on the front. Questions also existed on the color; #37 is dark blue, whereas the subject's color, although it appeared blue, had not been specifically identified.
The postmark, at the magnification level of the original posting, indicated source as Bern, but the date was not identified.
As to color, here is the subject followed by #277 and #490, both dark blue,



Here are two subjects identified as 'blue,' # 94 and #120.


Magnification of the back revealed no color identification. As previously reported, the color of the thread on the subject may have dissipated during the paper removal process. Magnification of 130x revealed no further identification.
High magnification of the postmark, however, appears to be the following, which, if correct, identifies the stamp, as previously assumed, to be a #37, postmarked in 1861, regardless of the color difference.

Final conclusion was that "still a great stamp, and a keeper." I'll settle for that.