Hate to necro this thread, but this document will probably be of use:

http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/datacomm/GA27-3093-3_SDLC_Concepts_Jun86.pdf

The reason the SCC was needed for LocalTalk is because LocalTalk uses IBM SDLC as its low-level transport protocol (hence the checksums, etc.)