18ct with line under t

[bread_baby]

I have this ring with typical english style prong setting set with old european cut diamonds. It doesn't have any marks except the 18ct mark. Because the mark has distinguishable features I was wondering if someone can tell me when this mark was in use. 18 ct underlined letter t. The image doesn't show the line because of the angle of the photo but it's there, A smaller t with a line under it.