C for Chicago indicates the Columbia Custom Office in Chicago–the hub for outside, independent label clients.
They did not actually press the records there. The Columbia pressing plants for 45s in the 1960s were Bridgeport CT, Terra Haute IN, Pitman NJ and Santa Maria CA. Records pressed at any of these plants will still have the ZTSC code if the paperwork went through the Custom Office in Chicago.
The metalwork could have come from either a Columbia source (hence a stamped matrix) or sometimes from outside mastering (accounting for the etched or scratched matrices). As long as the account was administered through that Custom office it got a ZTSC matrix.
There were other custom offices, Nashville (ZTSB) and Pitman (ZTSP).
Pitman also handled the admin for the vast majority of the Columbia the in-house labels too, so Columbia, Epic, Date, Okeh etc. in addition to lots of independent clients. ZTSP on a label or run-out doesn't necessarily mean the 45 was pressed at the Pitman plant though, just that it handled the admin of the process.
Each of the Columbia plants used slightly different type styles on the label copy, whether it be different font sets or different layout styles.