Complex sentence: According to Berkhof (2011), Berkov was "Director of the Admiralty Yard (Directeur van de Admiraliteitsscheepswerf)", but states that (at the moment) no original source material is available which conclusively proves that Berkov held a Navy Admiral rank.
Split into simpler sentences: According to Berkhof (2011), Berkov was "Director of the Admiralty Yard (Directeur van de Admiraliteitsscheepswerf)". Berkhof states that (at the moment) no original source material available is which conclusively proves that Berkov held a Navy Admiral rank.