Until not that long ago, Vatican service passports didn’t have birth-date fields. This caused havoc at some airports…
A few years ago, I was told that I should travel on my EU-country passport, and categorically not on the British one (which used to have a B permit linked to it). I’ve since moved the permit to the EU passport, mainly to avoid the language test requirement 
Someone mentioned IATA above as the creator of the passport standards. It wasn’t, it was ICAO the UN agency responsible for aviation. All of their decisions result in recommended practices (RPs) which members are encouraged to follow.
This allows countries, like Switzerland, to apply variations. Nations are encouraged to notify ICAO should they differ from the standards which most, but not all, do.
Recent updates to the standards recognising biometric data stored on chips were done by issuing new RPs. Countries are not obligated to follow them.
Yes, but the problem is exactly that, they are allowed to not follow the recommendations, or to follow them partially.
Take for example the passport ICAO standard specifying to put XX as the day or month when the exact date of birth is unknown. Many people naturalized eg in Germany, Switzerland have XX in their birthdate in their european passport.
Although at the border crossing the systems are aware, however, these people cannot get an ESTA, ETA, etc do online airline checkin, as these systems do not accept XX.XX as a valid date “birth date as in your passport”, meaning they have to go to an embassy and purchase visas to travel.
PS: 6 years ago, Switzerland agreed to give those people affected (mainly eldery refugees from east asia) dummy birthdays 01.01 so that they can have a “real” birth date on the id documents.
Just discovered one today - filling out some random crappy online form; and realised that Swiss passports don’t have a place-of-birth!
They have a heimatort (nowhere near where my place of birth is, but rather my wife’s parent’s hometown)… that will cause some interesting questions!
These are sovereign nations. Nobody, not even the UN, can tell sovereign nations what to do. But the majority do follow the RPs to the letter and the RPs give a way for states to notify exceptions.
The Swiss Lieu d’origin has a long history and was very important in previous days. Not so much now. My Lieu d’origin is the commue I was living the day I was nationalised. Absolutely irrelevant to anyone else.
FYI, Passports are the most standardized type of international documents in the globe. There is a MRZ code at the bottom of the first page, generally this MRZ code is split into 2 lines, and it is standardized across ~200 countries with the exception of a few. I have worked on data sets of Passports to extract information from these MRZ codes, they are pretty standardized. When they scan your passport these MRZ codes are read by the machine, that’s my 2 cents.
There are of course exceptions, diplomatic Passports or Passports of organization such as UN etc. Of course their are also biometric passports with a chip inside, but not all countries issue them currently, so at least all Passports have an MRZ.
I was suprised the Swiss passport I got in 2021 didn’t have the signature I provided for the ID card… there was a blank line I could sign myself! Not sure this has changed with the newer versions.
As the OP, just a quick thanks for all the comments and interesting discussion. I’ll be back before the end of the 6 month period (by just a few days) on a one-way flight. I will post again just to let you all know if there were any difficulties……..Cheers
Well I flew back in just before the 6 month ‘deadline’, on a one-way ticket. Had to leave again a couple of weeks later, and returned a week later. Came thru the Swiss auto gate at the airport - no problem at all, so all seems to be in order.