Riddle me this... Suppose you have a box of class T weights, and in that set are five 20 lb. weights. Say for instance, the weights are 19.999, 20.000, 20.001, 20.001, and 20.002 lbs. respectively, each with an uncertainty of 2 grams or about 0.0044 lbs.

I pull #1 out the box and take a measurement on a scale and it reads 20.0 lbs, and I report the uncertainty as 0.0044 lbs (plus local gravity, scale resolution, and whatever else goes into the combined uncertainty.)

Now suppose I grab a random one of the five 20 lb. weights, take the measurement, but don't know which weight was used. The scale still read 20.0 lbs, but now is the uncertainty 0.0044 lbs. plus an additional 0.003 lbs. (because that's the spread of all 5 weights)? How would I work that into the total uncertainty?