From photos in Susan Tobin's Wedgwood Glass book it looks like an RSW39-1, described as 15 oz - presumably how much it holds? So the designer would be Ronald Stennett-Willson And the horse is I think the Stubbs's horse. That is shown on a paperweight but it is the same little jasper disc as on this piece. presumably was attached to a number of pieces. On page 23 of the book it describes this tankard as having first been made for the Christmas gift market of 1971. delivered to stores from September on. It doesn't say how long this range was made for but I have seen quite a few around, so probably a few years I suspect.
Hope this helps.