Above is the project as it entered the shop. Framed side by side with an antique engraving of the Founder. I did not ask the client, but from the looks of it, this framing was done in late 1960s, well before the advent of archival materials. The mat was clearly discolored is spots from acids in the paper.
Here is the back of it on my operating table (yes, it is dual-sided with a mat opening to reveal the addressee), with more apparent discoloration.
The completed project. Front and back views.
Your most obedient servant. Photo by the pleased client.
PS. The image of the raw letter at the top of this post is a large format upload. Clicking on it will greatly increase its size and the text can be read. Enjoy!