Identifying Your Vintage Henry Griffiths & Sons Trilogy Ring - Appraisal & History

Question
I Have A Yellow Gold & Platinum Trilogy Ring Given By My Nan , Its 3 Diamonds Mounted In Platinum And Half Way Down The Shaft It Connects To Yellow Gold The Only Significant Marks I Can See Are ...... Letter G , Large Gap Then H.G & S (HENRY GRIFFITHS AND SONS OF BIRMINGHAM I HAVE DISCOVERED FROM RESEARCH) Then The Is An Equation ( For Example 1/4 but So Very Small That I Cant See Even With A Magnifying Glass What It Actually Says ) Then 18ct Which I Take To Be The Gold Value , Then PLAT Which I Assume Represents The Platinum Properties And The The Numbers 84 ?? I Need To Know The Age Of The Ring And Approximate Value If Possible For Insuring Purposes , I Originally Thought That It Must Have Been Made In 1984 Hence The 84 But From The Research I Have Been Doin This Seems Impossible As The Jewellers H.G & S Seem To Be From 1890 - 1958 ?? I Understand That This Is English So You May Not Be Able To Help x

Answer
The 84 may be a style or refference number the store may have used for Identification.
For the best appraisal in England see a Master Valuer FGA. There is one in Bristol who is very good whom Ive spoken to over the phone. I'm sorry I cannot remember his name.

Arthur DeMello GG (GIA)
http://www.nationalgemologicallaboratory.com