Gold/platinum ring with hallmarks from 1867?

by Myriam
(the Netherlands)

Good morning,

I am currently watching a ring on EBay.
The seller states it's "an antique Victorian Engagement ring set with a round, "English old cut brilliant" 3 mm Diamond; mounted in an Heart shape illusion Platinum setting with a heart shape open back.

The Platinum Heart Diamond crown measures a dainty 5.2 mm length,5.5 mm wide and 4.5 mm high and is set between scalloped edge and glittering tooled Platinum shoulders; on an 18ct rich yellow gold 1.8 mm wide shank.The interior of the gold shank is crisply embossed with a full set of British Birmingham 1867 Hallmarks; including the gold carat mark "18" and the Platinum mark "PLAT".The weight measures 2.37 grams.

Isn't 1867 a bit early for the use of platinum? I also think the style of the ring is more art deco than Vicrorian? Could it be possible that the shank was made in 1867 and the platinum was added later? And if that's the case, are there any ideas on how old the platinum part could be?

Thank you so much!

Myriam