I seem to remember a company down south having purchased a Big Twin on the US register attempting to put it on the UK one, they were told that the props were not recognised on the UK register so had to be changed, An enterprising member of staff found them fitted on a Spanish registered aircraft so the CAA had to allow it onto the UK Reg
This is a very real issue. There is no central database of approved mods in Europe and the vast majority of stuff that has been approved locally is never made use of - because nobody can find it.
The CAA has their AAN database, but there is no database for Europe as a whole.
This could be one of the great advantages of EASA.... when they get it all together.