Seems to reinforce my suspicion that "E" means 67-page.
I found this out from the US Govt pages on ESTA - the new P series passport has two letters, and this indicates the type.
https://help.cbp.gov/app/answers/detail/a_id/1749/~/esta---australian-p-passportsPA Regular Passports
PB Regular Passports
PD Diplomatic Passport
PE Official Passport
PF Frequent Traveller Passport (extended pages)
The following "P" series documents will not be able to submit an ESTA travel approval.
PC Certificate of identity
PU Titre de Voyage
PW Document of Identity
PX Emergency Passport
PZ Document of Identity