Help needed with White Star Line Ismay Imrie logo

[Cornish Nige]

Hi all

I am trying to find more information about a White Star logo that is on an old envelope I have.

It is not the usual red with white flag but a slightly "masonic" looking one.

I want to sell the envelope but in doing research have become fascinated with the whole Titanic subject.

Could anyone shed any light on this mystery for me ?

Thanks