Hello Geri,
Another guy from IL. I used to live in Peoria until about 7 yrs ago. That doesn't look like Venetian plaster. The silver/copper flecks you talked about, did they look like little rust spots? A lot of older plaster used sand that wasn't washed well and often came straight from a riverbed. Depending on what was in the sand/soil you get some staining that would bleed through when the wall was finished. A good quality stain blocking paint should take care of it. Schellac or an oil based primer usually do the trick for more stubborn stains.
As for the 'recipe' you saw it's hard to get in the head of the guys who wrote it. A common mix from the old time plaster guys would have been 3:1:1 sand : Portland : lime. Cream could be the color of material used or even for Cream of tartar that was sometimes added to slow down a set, or it could have been the sand color, or something totally different. The buff part of anyone's guess. Rather than trying to figure out and duplicate what they did I'd go with stuff you can get ready made now.
For anything that needs to be 3/8+ I'd use Structo-lite or gypsolite. On that I'd use diamond or imperial veneer base and finish coat with diamond veneer or Kal-kote smooth.
Hope your project goes well.