As Curtis explained in a previous comment, I inserted the Calculate Value into the sub-model.
The sub-model iterates the field values of the feature class and gives the feature class path stored as text in an attribute.
The Calculate Value tool takes those paths and makes the output a Feature Class>
The Collect Values tool gathers the calculated value of the output of the iteration and that is the output and model parameter called "List of Contour FC Paths" below.
The Calculate Value settings are below that.
The sub-model was added to the Merge Contour Tiles model, the results merged and then dissolved.
It is necessary to put the collect values as the end of the submodel because
if you put the merge in the submodel it will iteratively merge each output,
making the Collect Values an un-useful annoyance.
After the Merge the contour lines were dissolved by elevation attribute, which is an integer.
Not shown is the next step of adding index fields using/calculating the even remaindered intervals
using the Modulus or Mod() function.
I have a script for generating the area of interest from the extent of a map.
This extent polygon is then used to select the footprints from the index of tiles feature class.
The selection becomes the input for this model.
The output of the model is a contour line feature class subset only to the property for which the user is making a map.
Many thanks to Abdullah Anter (with D. Patterson and D. Wiens replies) for his prior thread which helped greatly
in my task of identifying my problem.
Cheers Curtis, it worked!