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?)
Looking at the specifications in http://frobnitz.co.uk/zmachine/infocom/, I see them mentioned in both the xzip and yzip specs, but not in the zip and ezip ones.