Due to the date mentioned as part of the rubber stamp, I believe that this is a live recording and not just a pirate of officially released tracks.
Matrix says: NRV-6-S1013
It does remind me of the earliest WCF/pre-Berkeley Records phase, when they used a combination of letters and numbers before switching to an all number-based system. here is an example, LIVE BAND by the Band:
Back to the Chicago album: The seller set an auction price of $8 but this otherwise unremarkable LP – especially for the non-bootleg aficionado – sold for six times that much.