What Tommy said. SC and ITT phones were almost exclusively sold to independent telcos. Most independents were on shoestring budgets and had to rely upon frequency-selective ringing in an effort to avoid the expense of plant improvements. It was cheaper to buy phones without ringers and stock them on the trucks with a selection of different ringers that were field-installed based upon the installation requirements.

Your location indicates that you are in NC, a state that is full of independent telcos, so your find doesn't surprise me at all.

Ed Vaughn, MBSWWYPBX