Finally! I found the solution to this intriguing problem.
If you open the object for editing you will see the parameter "gs_masonry_arch_pen" listed under the "3D Representation category" but the category title is set to "hidden" so therefore it doesn't show and any parameter under its hierarchy doesn't show either.
So to change this you need to save the object under a new name, then you will be able to modify it.
Right now it is within the LCF file so you cannot make changes, this is why you have to save a copy of it and modify that.
If you want to modify the one in the LCF, then extract the LCF, modify the part, then recreate the LCF and replace the original AC LCF with it.
So, when you are in the Library Part Editing Window, find the "gs_3D _representation" parameter and click its "Hidden" toggle to turn hidden off.
Save it and the 3D Representation category along with the Masonry Arch Pen parameter will be visible among your parameters.
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