As much as it is hard to believe, the information provided to you above was not an opinion but derived from the chronicles of English hallmark guides. Here is an image from the pocket Jackson book of English Hallmarks. The anchor is a Birmingham assay office, this is a page from the Birmingham assay pages. The letter mark is the date mark and is generally matched by the style of the font and the outline of the frame. You have the added fortune of having the Jubilee mark that you will find on this page indicates that the date of 1977 can have that mark but no other can. So look for yourself that it is not an opinion but researched documentation. No disrespect at all taken or given, this is a learning forum. Here is the page...

Chances are that the ring was a recent gift that your grandpa enjoyed immensely and wore all the time. 1977 is still almost 30 years ago which is a long time. Treasure it as the gift of memory it is.