Sinclair held the title Earl of Orkney (which refers to Norðreyjar rather than just the islands of Orkney) under the King of Norway.

The authenticity of the letters (which were allegedly rediscovered and published in the early 16th century), the exact course of the voyage, as well as whether it even took place, are challenged by historians.

Most regard the letters (and the accompanying map) as a hoax by the Zenos or their publishers.

It is supposed that he died shortly after that although his son did not take the title until 1412. In the 1980s, modern alternative histories of Earl Henry I Sinclair and Rosslyn Chapel began to be published.

Popular books (often derided as pseudo-history) such as The Holy Blood and the Holy Grail by Michael Baigent, Richard Leigh and Henry Lincoln (1982) and The Temple and the Lodge by Michael Baigent and Richard Leigh (1989) appeared.

tried to relate this to the name "Sinclair", but "Prince Sinclair" is not normal usage, while "Prince of Orkney" seems a better fit, and Frederick Pohl points out that the "Z" could have easily come from a misreading of the cursive "d'O" in "d'Orkney".

In his letters, Antonio Zeno describes a spring of pitch running down to the sea.

Initially trialling de L'Arde as Captain of Orkney, King Haakon VI of Norway was quickly disappointed in de L'Arde's behaviour, and sacked him.

On 2 August 1379, at Marstrand, near Tønsberg, Norway, Haakon chose Sinclair over Sparre, investing Sinclair with the Jarldom.

According to Lomas, Sir William, the chapel builder, is also the direct ancestor of the first Grand Master of Masons of Scotland, also named William St Clair (Sinclair).