I tried a few months ago and couldn't find a way of retrieving the six-digit number without calling IB. There are (or were) a couple of oneworld airlines' websites that allow retrieving a booking with a ticket number and surname, but they didn't work with an IB ticket number.

On a side note, even Iberia's own website thinks that an Iberia PNR should consist of six digits rather than five LOL!!

On our website at iberia.com, by accessing Bookings management. You will need to enter the 6-digit booking code and a passenger surname.
Through our Serviberia service, on + 34 901 111 500. Choose option 2 on the voice menu and then give the 6 digits from the booking code and a passenger surname.

https://help.iberia.com/frequently_a...rm-my-booking/