What does 373 stamped on gold jewellry mean?

I just got a pair of earrings that have a hallmark that is too small to read and the number 373 stamped on the studs. They are gold with a square diamond-like stone under a white gold bow. I say “diamond-like” as I don’t for a moment believe they are real diamonds. They are from Fraser Hart jewellers in England. Any help would be appreciated.

If you look closer, it should read .375

The standards of fineness are:

Metal: Compulsory Mark
9ct gold 375
14ct gold 585
18ct gold 750
22ct gold 916
99% pure gold 990
99.9% pure gold 999
800 grade silver 800
sterling silver 925
Britannia Silver 958
99.9% pure silver 999
85% Platinum 850
90% Platinum 900
95% (UK standard) Platinum 950
99.9% pure Platinum 999

Gold Diamond Square
Gold Diamond Square
Gold Diamond Square