When we purchased the property, from new, a Plot number on the named comunidad was used to identify it.
Over the years the Town Hall named the roar (Calle) and gave us a number. A bit more time went by and the Post Office came along and as we were on a comunidad they said we must all have a communal post box . Each property have an individual box and number.
The post would only be delivered to the box number. No box number on letters etc would mean no post delivered !
So I guess the answer may be to put all the different 'numbers' on letter etc.
Good luck with that !
Have fun it's Spain.