I noticed a 13C KTU being auctioned off, but couldn't locate any BSP for it, maybe from a 1A1 key system? Anyone know where I could find the BSP for this? Thanks!

Here it is: Click here

or more what you're looking for

BSP 518-112-422-i01 Page 2 Fig. 2-13C Key Telephone Unit, Automatic Signaling Intercommunicating Circuit

The 13C is 1A (not 1A1).

Sam

As the documents state.