From the Z-spec 1.1:
The opcodes EXT:5 to EXT:8 were very likely in Infocom’s own Version 5 specification (documentary records of which are lost): they seem to have been partially implemented in existing Infocom interpreters, but do not occur in any existing Version 5 story file. They are here left unspecified.
Emphasis mine. Is this still true? I thought Infocom’s specifications were recovered and published around the same time as the source code.
(And if so, do we know what those four opcodes did?)