Hello Wurger and thank you very much for your kind reply!
About the aircraft's serial, I believe I have a good reason to say it was EN671.
I searched on "nationalarchives.gov.uk", looking for RAF 682 Squadron "Records of Events" and "Summary of Events", both about September 1944.
These are the respective results, taken from an abstract of the whole files.....
View attachment 892601
.....Spitfire EN671 flew on the 2nd, on the 5th and on the 9th for operational duties.
And it flew also on the 1st of September....
View attachment 892602
.....for administration duties.
As you can see and read, no records about EN677.
One more detail that supports these words of mine is that some sources on the net say that the picture of the "white G" posted by you has been probably taken at San Severo airfiled (near Foggia, a city in the nothern area of Puglia). Please read the note about the 1st of September happenings.
This said, I'll think about the solutions you suggested me to adopt to reproduce the famous "white G" letter onto my model.
Thank you very much again for the help you gave me. I really appreciated!
Best regards.
CARLO