The thresholds don't alignment with Google maps. I will try to check Europe Datum 1950.
Don't rely on Google Maps as your benchmark.
When the coordinates was re-calculated from local datums to WGS84, may be coordinate Z (altitude) was omited ?
Very unlikely - the basic method of conversion uses the Molodensky parameters that includes the X.Y & Z parameters. I did most of NZ & the SW Pacific islands in the early 90's and used the Bursa-Wolfe 7 parameter method where available.
The accuracy is still dependent on the source data. The last time I used some data from the Russian AIP, I found Google Earth & The World Aeronautical Database (online) far more accurate...