Update:
I found this site from the Rio Grande Modeling & Historical Society
http://www.drgw.org/data/freight/1960/index.htmwhich answered the remaining questions. No, the RG never had a car numbered 64188, and the group 64150-64199 was unused. So, for a freelancer, this car can be used as a "might have been" without conflicting with a real car series. As far as I know it's the only car available in this paint scheme, which seems to have been rare.
Here is the result of minimal upgrading. Other than the MT couplers, nothing was purchased for this project.
http://imageshack.us/a/img90/7258/drgw64188afterupgrades.jpgThe changes were:
Paint the ends black. Very simple, mask the sides and spray away.
Replace the "New" date. It was printed much lighter than the rest of the data and just didn't look right.
Add a "Blt" date. Both came from custom decals bought years ago for my modern narrow gauge.
Replace the modern consolidated stencils with the early variety. That used leftover MS data decals.
Add the "Plate C" stencil, also from MS. I have a lifetime supply of the MS data because, for many years, they were the only source for ACI labels.
Add end initials and numbers. These came from CDS narrow gauge dry transfers, bought for the dimensional data, which went on old cars for my narrow gauge.
Grunge up the wheels a little.
A very simple project that makes a toy-quality car usable, at least for a non-D&RGW modeler.
Again, thank you for the information, and if I ever find an undecorated model, I might do the WP.