The Bit | Effect table for Tilemap Transparency Index NextReg $4C on page 90 shows bits 7-5 - "Reserved, must be 0" & 4-0 - "Index of transparent colour into tilemap palette"
The documentation at https://www.specnext.com/tbblue-io-port-system/ shows as:
bits 7-4 = Reserved, must be 0
bits 3-0 = Set the index value (0xF after reset)
The Bit | Effect table for Tilemap Transparency Index NextReg $4C on page 90 shows bits 7-5 - "Reserved, must be 0" & 4-0 - "Index of transparent colour into tilemap palette"
The documentation at https://www.specnext.com/tbblue-io-port-system/ shows as: