According to wiki the name of the hotel was incorporated in 1985 as
In 1974 the company's hotel interests expanded further through the acquisition of a 49% interest in The Oriental, Bangkok. The Oriental, built in 1876, was already a legendary property and acknowledged as one of the world's great hotels. Through the management of both The Mandarin in Hong Kong and The Oriental, Bangkok, the Group was in an unusual position of having two "flagship" hotels whose names represented the best in hospitality. In 1985, the Company rationalized its corporate structure by combining these two renowned properties under a common name, Mandarin Oriental Hotel Group.
The actual ref letter: The Oriental..... Also the the date 1876 - 1988 is ridiculous, so every year they update the current year?! No....
I believe the new incorporated name in 1988 would be updated on all letterhead and not missed by this worldly hotel.
https://en.m.wikipedia.org/wiki/Mandarin_Oriental,_Bangkok
The logo is not the logo used at that time, nor ever.
In 1974 The Oriental in Bangkok, which was already acknowledged as one of the world's most legendary hotels was partially acquired by the Group, giving the company two "flagship" hotels whose names represented the very best in hospitality.
As a consequence, the two famous hotels joined to create the brand Mandarin Oriental Hotel Group under the renowned fan logo.