Yesterday I happen to be in Burbank California for work and was close to "German Auto Tops" in North Hollywood so went over to their shop to look at interiors and tops. Here's how John at German Auto Tops explained it.
Stayfast - least cost, basic top canvas product. I saw the roll of material and it clearly looks different. Both inside and out are slightly different then a Mercedes top. It still looks good, but on close examination, one can see the difference, especially on the inside as the finish is plain.
Twillfast - also known as Sonnendeck. John said they are the same material and both names are used. This material is very similar to Sonnenland according to John. The colors are slightly different. Tan is not the same Tan as Sonnenland. The texture and feel are almost identical. The Twilfast/Sonnendeck felt a hair thinner in my opinion, but very marginally. The inside is almost identical as it has the dobby weave lining (a sort of herringbone pattern), as the Sonnenland.
German Canvas - also known as Sonnenland. John explained that this is the closest to original. For me it was not easy to tell this one apart from Twilfast, but looking and feeling closely you can find the difference. I have a swatch of each that he cut for me.
This is how German Auto Tops explained it ... I would not be surprised if other shops explain it differently.
I can say one thing about German Auto Tops is that they are a VERY busy little shop and were in full production mode when I visited with probably 8 guys building tops or other interiors. No affiliation as I was just trying to decide on materials and possible color change so stopped by.
